Just to pipe in with some first-hand experience with a very similar truck ('49 3804)... if anyone is still interested at this point ...
A big foot stomp to everything Jerry and Coilover said.
If you're changing the engine anyway, upgrading to a 261 is a great way to go. That's what I did. I did two very simple upgrades that were as easy as they come with *zero* modifications required. I now have plenty of power in stop and go traffic, and the ability to cruise 60-65 effortlessly, plus ability to dash to 70-plus when needed.
Upgrade One -- 261 install. For zero modifications required, look for a 261 from the 1954 mold. These engines were built for the '54 and '55 1st series trucks, so they have the provision for the front engine mount and the high-mounted water pump. If no front mount, merely use the front plate (what the timing cover attaches to) off your old engine. Look for a 261 with the casting code of 3733950. NOTE: Some of the 261's with this casting code will have date codes later than '54. Mine did. Threw us for a loop on the bearings. But it was because it was built as a crate motor for the '54 and '55 trucks. You can read about the group effort to rebuild this engine (under Jerry's tutelage
)
in our "Rebuilding the 261" tech tip.. Some interesting things we found out about the engine so read the tech tip BEFORE you order parts ... like bearings ...
Using this engine requires no mods -- it just bolts right in. You can even use all the peripherals from your old engine, except the manifolds and carburetor. No need to move the radiator, fiddle with engine mounts, reroute accelerator linkages, or drill holes for water pump relocation plates or any of it. If you use a '55 or later 261, it will require some mods, as will a 235.
Also, when trying to identify the '54 mold 261... you will read on this site about the "Captain's Bars" being an identifying mark of the 261. This is not entirely true. The '54 mold did not put Captain's Bars on the block, so don't bother looking. Look for the casting code and don't let the absence of Captain Bars throw you off. But if you *do* see the Captain's Bars, it will probably mean that you have found a 261 that will require some modification to work in your truck.
Upgrade Two -- Replace the rear axle third member with a 4:10 ratio from a '68 to '72 C-20 (with auto tranny). That upgrade is covered in the TechTip Coilover referenced. Oh, as a 3600, are you running 16 or 17-inch wheels/tires? I'm running 17's on my 3800. If you run 16's, you might not get the speed bump I did.
Note to remember -- A lot of advice given by 3100 owners is well intentioned, but remember that those trucks are different than our 3600 and 3800's -- we have the HO52/72 rear axle, which makes the ratio improvement an insanely simple upgrade. Their rear axle gear upgrade is not so simple
Neither is finding/adding overdrives or any of the other add-on speed equipment. The very simple HO-52/72 3rd member swap is as simple, and relatively inexpensive, as upgrades get.
Lastly -- Jerry covered it well, but ... we are deadly serious about the brakes and the steering -- neither of which were expected to deal with modern highway speeds, so make sure they are tip top condition before you break the "AD Barrier" of 50 MPH.
Here's a shot of my 261 all nestled into the 3804 engine bay. Note the high water pump. The engine color (Detroit Diesel Alpine Green) and the Rochester Carb are the two main "tells" that this isn't the original 216. I did use the kit to mount the 216's valve cover -- something that I am now starting to regret. It looks cool, but it blows oil all over the place
Hope we've helped!
John
