Boo's got some great suggestions. These are things that can be done without pulling the motor and should wake it up somewhat.
But remember...you've got a heavy truck (relatively) and a small motor (relatively.) The 283 isn't known as a torque monster at all. So whatever performance you get out of it won't be earth-shattering.
However, as Boo has alluded to, the motor's just an air pump. More in, more's gotta come out, equals more power. All of the components play their own part. If you do the intake first, the flow will be restricted by the heads and exhaust. If you do the exhaust first, the intake and heads will be the bottleneck...and so on.
The cam also plays a part in it and an "RV" type cam wouldn't hurt in a heavy truck like yours. I wouldn't suggest buying a cam with lots of lift and duration, because they're meant to add horsepower in the upper rpm ranges..and the lower torque range can suffer...or not be taken advantage of as well.
A quick way to start would be by getting a set of cheap headers and Purple Hornies. I've used 'em and they're a hell of a lotta fun. But if you're in a quiet residential neighborhood, maybe a nice set of 2.5" dual exhaust pipes with turbo mufflers will do a better job of getting the exhaust outta the way without a lotta noise pollution tickets.
Personally, I run a 292 I6 with split headers and Cherry Bomb glasspacks. Yeah..it's old skool and that's exactly what I wanted. I also run a 625 CFM Carter AFB carb on it and it runs great.
Anyway, once you're done the exhaust and gotten familiar with some of the things under the hood, you might want to attack the carburetion and intake manifold issue, just to get familiar with doing it...cuz if you change heads and cam later, you're gonna do it again. I'd suggest an Edelbrock 2101 Performer manifold. I don't think you'd gain a lot by going to the next step up, the RPM.
As for carbs, I'm partial to AFBs...Carters or Edelbrocks. A 1403 (500 CFM) or 1406 (600 CFM) with electric chokes (I need the electric choke where I live) would be what I'd go with. I've used 1406 carbs on 305s without it being too much...and if you ever decide to replace the 283 with a 350, it'll be just right.
Then the cam and heads. This is a little more involved, but it's not all THAT much harder. Those cast iron heads are sure heavy to lift outta them big ol engine compartments. If ya got an extra $1300-$1500, you could get aluminum heads...but then that'd be sorta silly. If you can find the Power Pack heads, those would be great. I'd have a machine shop do a valve job and replace the valves, guides and seals for a long-term solution. I've cheaped out on this before and regretted it.
If yer gonna replace the heads, for sure do the cam, lifters and use a good double-row timing chain and gear set. This might just do as much to make things more consistent than many of the other items. If yours is original, I bet it's stretched all to hell. Btw, Summit Racing sells a cam and lifter kit for $80 (I'd go with an 1102 or 1103 version.)
When ya get that back together and running, you'll prolly have around 220-250 hp. Not enough to stop the presses, but prolly a lot more than you have now.
Finally, if you wanna see the smoke and oil and parts start exiting thru the oil pan, you could always add a 150hp shot of nitrous. That'd be kool for a while, but I doubt that the motor would live very long that way. Like maybe 4 or 5 passes down the quarter mile if you've got a worn bottom end. Btw, this type of cataclysmic event is known as "oil pan failure." That is, the failure of the oil pan to hold all the blowed-up parts that are rattling around inside the crankcase like shrapnel.
Now...if I was gonna do it...I'd pull the little 283 out, set it over in the corner of the garage as a doorstop and build up a 400 (406 or 408 with a .030 or .040 overbore) smallblock, Vortec heads, Vortec RPM manifold and 750 CFM carb. Ohh...and a nice healthy torque-monster cam. I like 400 motors.
There are several stories of engine build-ups that I've done over the past few years on my website. Unfortunately, I've had to sell most of the cars in the last year because of medical problems. But if you go to the "Dearly Departed" section at the bottom of the main menu page, you'll see several of them that have had motor built...and you can also see the 2 remaining V8 vehicles I have run 400 smallblocks: the 80 El Camino and the 85 Monte Carlo (alleged salt flats race car.) The url for my site is in my signature.
Good luck with your project and I hope you don't mind my wordy response.
