Here is what I would do with your goals & motor etc.
1. Have your salvage yard do a salvage yards computer search for a gbody MCSS whole rear ie drum to drum. It will be either a 3.42 or 3.73 and may or may not come with a posi. You can also check all the gbody forums as there is like 6 to 8 large forums that I know of. Also Craigs list and carparts.com. Between these you will find one for a good price/deal.
It is usually the cheapest easiest & overall most idiot proofed way to do it. You can swap it out yourself no need for a shop, special tools, shims, & knowhow like changing the actual gears. You should still change the fluid.
2. Get CKPerformance.com 's excellent shift kit. (About $100ish package price) 12" 2,800 stall lock up torque converter ($299) lock up switch, & speedometer gear. (for your rear gear change) Make sure you get with your shift kit what you need to change WOT shift point to 5,200 rpm or at minimum 4,800 rpm. You should have like a 4,400 WOT shift point now.
3. Get electric fans from a salvage yard. Do a search on here there are several good ones. You will also want to upgrade / replace your altenator and make sure you have a good ground as well.
4. Use your performer intake as well as the rest of the stuff you have. Get a 4bbl Q-jet carberator. It will get better mileage & performance than the two barrel. Have a Q-jet shop / specialist get you the carb or rebuild yours and have them put the right jets and hangers in for your engine cam & rear gear ratio. A little shopping on the net and you can get this done very reasonably without the hassel frustration and learning curve of trying to do it yourself. IMO
5. Headers are a good bang for the buck. You don't have to ceramic coat but ounce of prevention is truly a pound of of cure here. At least sand off the junk factory paint that they put on just to keep them from rusting before they get to you. Spray them with several coats of good header paint. Make sure that at least between the first and second coat to let them dry / cure a full day before adding the second coat. Also get good gaskets and throw any junk gaskets that may come with the headers away. I recommend
http://www.remflex.com gaskets and they will not leak, burn out, and you won't need to periodically re-tighten your header bolts. The extra time & minimal expense to do it right the first time is way easier than the headaches cost & time of doing it over. Mini starter only if you want / need to for easier header install.
Iceman has cheap & easy dual hump crossmember. Add an easy inexpensive H pipe. Usually about 8 to 10" back from header collectors. Maybe buy a Summit or other Header back system. Cheap Summit or other "straight thru" type mufflers. The largest case that will fit will give you the best flow without being too loud. The H pipe and tail pipes will keep the sound down and no drone or excessive noise like Flowmasters. Best bang for your performance buck and great sound. Naturally if you prefer the patented flowmaster sound and don't mind a little more restriction / flow loss or cruise drone and or want the loudest use flowmasters with turndowns.
6. I would do a cam & lifters kit AND get new performance springs with them. Your factory springs were too weak when new let alone now via age as much as mileage. They would suck bad with a new cam. My pick would be Lunati Voodoo 60801LK
http://www.lunatipower.com. Lunati says this is the BEST cam choice for a daily driven street performance 350. It is an Oldsmobile specific cam and not an outdated leaves much to be desired generic grind. This is a lot less cam than 86 Black 442 is useing in his car. It is also well suited for your low compression 71 engine. You could get away with another bump up with your converter and gears but honestly this would be my pick for your overall car & goals.
This car will be fun run & sound great and can still get good mileage. It won't be dangerously fast just a fun reliable enjoyable driver. Probably with decent traction a solid 14 second car.
