As Ben (Newbie) wrote, I also have JBA shorty headers and they do fit great and seal up real well. I installed them on my new 383 because I kept cracking my stock exhaust manifolds. They did help power, seem to add torque which is great as I tow with the 94 Suburban. As for engine compartment heat, not too bad. Last summer we towed in 100 degree heat so I removed the inner fenderwell splash guards and the compartment stayed fairly cool. Now that it rains everyday, sheilds are back on. I think the heat in engine compartments is over rated. I began towing in 1988 with a 1965 Chevy Van (still have it) in which I installed a 292 6 cyl stroker. I have full length headers on it. As the engine is between the seats in a doghouse, and setback about 3 feet from the front of the truck, engine heat is much more of a concern than standard vehicles. However, not enough to cause problems. Towed over 90,000 miles with no damage from heat.
As for bolt on (or off) up grades for a stock 350, first thing I did was get rid of the air intake box that reduced the intake pipe from 4" to 3". Now run a 4" dryer duct directly from the fender to the air cleaner. Also added a 3" air intake from driver's side fender to air cleaner. Engines need all the air they can get. Next, dual exhaust, that REALLY opened up the power. Can't stay with that stupid Y pipe and get any power. Two big truck mufflers and 3" exhaust and tail pipes. Kinda loud but not inside the truck. I don't have any cats but if needed could use a couple if I had to. Live in a rural area. Of course, tune up, new AC plugs wires, cap, rotor (got to be AC). Since that didn't make the low compression 350 powerful enough to meet my trailer towing needs, had a remanufactured 383 stroker installed a year ago. Now I have power, and still use two air intakes, dual exhaust and JBA headers. I can take some mountain passes in 3rd gear and cruise on the level in 4th. (No K&N air filters)