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I've been contemplating putting a set of roller rockers on my 327, but I have a few questions.
What stud size & ratio should I use & whats the difference? How much extra hp can I realistically expect?

None, if that is all you do.

Well it already has a Performer intake, Edelbrock carb, headers & a mild cam. Are you saying that there is no added benefit from adding these?

The effect will be minimal but using roller rockers will reduce friction and if they are well made ones they have a more accurate ratio than the stamped steel rockers. This improves the efficiency of the valve train and may realize some power increase. I used some Comp Cam Magnum roller tip rockers with a 1:52 ratio identical to these http://www.compcams.com/information/Products/RockerArms/MagnumRoller.asp

You can find them on ebay all the time.

Thanks Houston, but that brings me back to my original question as to the ratio & stud size. What's the difference?

Sure is nice not to have to adjust stamped rockers, though. Rockers need adjusting from time-to-time.

There are two stud sizes I am aware of, 3/8 which is what most stock pressed in studs are and 7/16 which are found in higher performance engines and are generally of the screw in type. You can retrofit heads from 3/8 to 7/16. There is no performance gain associated with the stud size that I can determine. The rocker ratio determines how long the valves stay open and care has to be taken to ensure an increase in the ratio does not cause a valve to contact the piston. I do not claim to be an expert on this so I would suggest doing a google search for more detailed information. Unless you are building your small block to a high compression configuration the 1:52 rockers would be more than adequate for your needs. I am using them inconjuction with an Edelbrock performer cam in my 350.

This style of roller rocker does not need periodic adjustment. There are some others which may if used in a mechanical lifter valve train. The hydraulic lifters do not require this adjustment once they are set correctly.

I run the Comp Cam roller rockers on a big block Chev, and they are 1:7 to 1. You do not want to run the big block rockers on a small block engine because you may develop enough extra lift so that your valves start kissing your pistons. This is not as much fun as kissing at the drive-in, and can be more expensive.
Bob

OK, so basically I should be safe if I were to get a set with 3/8 studs & 1:52 ratio right?

The lobe lift multiplied by the rocker arm ratio determines valve lift. The use of a 3/8 stud as opposed to a 7/16 stud is diameter, but you cannot interchange rocker arms. If you were to put 7/16 rocker arms on a 3/8 stud, you will create a lot of extra play and probably shear the stud off. If you know the valve lift and the rocker ratio, you can cross multiply to determine the change in valve lift using different rocker ratios.
Ex. 1.52 rocker with .503 lift and you change to 1.6 rockers. Multiply 1.6x.503 and divide by 1.52 and valve lift will increase to .529 by changing from 1.52 to 1.6 rocker ratio.

I have 1.6 ratio rollers on my 327. Motor seems to rev easier after I installed - maybe just my imagination !

Stock ratio is 1.52:1. Rollers are available in ratios from 1.5 through 1.7. Stay 1.6 and below.

britcon, the higher ratio rockers will change the cam's effective lift therefore changing power.

Bruce, besides reving easier, did you notice any other significant changes?

You may increase to 1.7 ratio rockers, but you need to verify the valve train geometry. This may cause you to adjust the pushrod length. Even with the 1.6 rockers, your should check the valvetrain geometry. You will probably make a more noticible power increase by adding 1.6 rockers to the exhaust side only. Many engine builders mix different rocker ratios to acheive a consistent power out of each cylinder. Since the outside cylinders are a longer overall distance, some will increase rocker ratios on the outside cylinders only and then increase just the exhaust rockers on the inside cylinders. You may need to experiment to acheive the best power balance. When you changed to 1.6 rockers, you will increase overall valve lift without increasing duration and the same goes for the increase to the 1.7 rockers.

Mobile: Valve train seems quieter. Had some bad rocker noise prior to replacement. Watch the clearance on your valve covers. I have Edelbrock finned aluminum on mine - no issues.

I have run many , many sets of rollers over the years. Couple key points- If you are happy with your cam, then stay with the same ratio. If you go to 1.6 rockers it will be like going to a more radical cam. Your idle is is going to get ruffer and it may cause you problems. So don't do it unless you want that affect.
Don't even think about switching to 7/16th's studs. This isn't necessary until you start running BIG lift cams or a roller cam. It's the kind of stuff you see at the drag strip idling at 1500 rpms. Its not a "just drill and tap" switch. If you need 7/16 studs in a head then you are at the level that you should run stud girdles. And in order to run girdles you will have to pull your heads and send them to the machine shop to have the holes drilled by an alignment fixture (makes all holes perfect in line for the girdles). Of course girdles mean you need polylocks and that means tall valve covers to clear all that BS.
Stay mild and you don't have to deal with that crap.
There is a performance difference and several benefits to roller rockers. The biggest difference is they will eliminate much of the valve guide wear associated with the side force caused by stamped rockers. (as they slide on a layer of oil back and forth across the tip of the valve). Since there was friction where the stamped rocker rotated on the nut you will also gain that as power. They are quieter. I got caught a couple years ago running them in a stock car class where they were not allowed. I pulled them and came back the next weekend. I am telling you from first hand experience. I COULD TELL A DIFFERENCE! It wasn't alot but I could tell my motor was not pulling as hard as it passed 6500 heading toward 8000 at the end of the straight away. Now you might say "what's that got to do with my stovebolt?". It doesn't, but if I could feel it, then on a dyno it would be measurable even at 3000 in your stovebolt.
In other words they will eventually pay for themselves at the gas pump.

JMO

SuperDuke "strongly recommends roller rockers"