The simple way to figure out the rear end ratio is to jack up the rear end of the truck (block front and rear of the front tires first), take it out of gear and then make a mark with chalk or something (yellow tire marker, piece of tape) like it on the drive shaft.
Then observe the driveshaft while you turn the rear wheel one complete revolution. If the driveshaft turns about 3 complete revolutions for one revolution of the wheel, there's a good chance that it has 3.07 gears...or something similar. If it turns about 4 times, it could have 3.73, 4.10 or something like that.
This is just a ballpark method of determining it, but it's always given me a good idea of about what gear ratio I'm running.
If you want to know exactly, remove the rear end cover (assuming it has a rear cover plate) and count the number of teeth on the ring gear, then count the number of teeth on the pinion gear. Divide the ring gear number by the pinion number and it will tell you the exact numerical ratio.
Then put the rear end cover back on and re-fill with gear lube. After that, you can shower about 14 times to get the smell of gear lube off your body. Burn your clothes, too.
You've mentioned getting a fancy exhaust for the truck. If it already has headers and dual exhaust, changing the type of headers and mufflers won't give you much performance increase, and going to "the largest pipe possible" may actually decrease power slightly.
You didn't mention what kind of carb you have on it. This may be the area where you can pick up the biggest gain, especially if you only have a 2 barrel (as Bill mentioned( on there now.
Now if it was my truck, I'd pull the 283 out (they're good motors, but they're not the most powerful smallblock you can get by a log shot) and replace it with a 350 or 400 smallblock. I'd use good stock heads (461s or 462s or even 882s) or possibly cast iron performance heads like Iron Eagles. I'd also use an intake manifols like an Edelbrock RPM and a 600 or 750 cfm carb. I like the Edelbrock 1406. It's a 600 CFM with electric choke. I've got 'em on 3 out of 4 of my cars I'm using an E-brock 1411 750 cfm on the other one and will eventually convert it to a Demon 750. The other thing I'd do is use a camshaft like the Comp Cams 268 and either run a Pertronix kit in the old distributor or replace the whole thing with an HEI unit.
If you started with a 350 and did all these mods, you'd end up with something that would make about 300-350 hp, give or take a few...especially depending on the heads you use. If you started with a 400, you'd get 350-400 hp, again depending on the heads and cam.
I've built many motors like this and they're real good for a street cruiser. But they aren't economy motors...although I got 17 mpg in my El Camino with 406 on a trip a couple of years ago. But at that time it had a 2.73 gear. Now it has a 3.73 posi rear end and desperately needs me to install the spare TH200-4R overdrive tranny into it. It gets about 8-10 mpg now.
You might want to check out the engine buildups I've shown on my website. The url's in my sig.
Good luck with it.
