It's a 302 with low compression and low flow heads, so he might see a 10% gain with headers only but I'm a firm believer that after any modification that affects the intake or exhaust one should always tune/jet the engine if they want to see any actual gains from their modifications. In this case of a better flowing exhaust via headers, the engine will run a little more lean so some compensation would be in order. I haven't seen the factory manifold on a '76 truck 302 but it would probably be pretty restrictive, like any factory truck manifold ever. Headers would free things up but move the powerband up higher.

 

I recently changed my manifold gaskets. The original one had literally deteriorated, leaving a gap. I think just replacing the gaskets, increased my driveability and throttle response. But, idk if thats true, or just me thinking that it did.

 

Exhaust leaks are definitely a bad thing. You're not going crazy. I believe that you did actually notice better driveability. Leaks let exhaust out early, marginally dropping exhaust temperature and velocity. They also let air back in after the positive pulses of the cylinders have passed. This further cools and dampens pulses in the pipe. The change in temperature and weakened pulses change the time and intensity of the negative pressure pulses that help scavenge exhaust from the chambers during valve overlap. That means more exhaust and less fresh charge in the cylinder.

I have no collector gaskets at all on my truck and all the welding on the exhaust was done terribly. Holes everywhere. Thanks, previous owner! Because of this, I have quarter throttle hesitation so much so that it'll stall the truck out if you aren't familiar with it. I know for sure that this leak is screwing things up. My exhaust pops on deceleration, a sign of a lean mixture, but it bogs on acceleration, a sign of a rich mixture. The engine also smells rich but I know if I lean out the mixture any more than I already have, the RPMs will drop too much. Basically, I'm lean at one end and rich at the other. It's messing with my driveability, performance, and fuel economy. Pretty soon I'll have the entire header-back exhaust redone so that'll solve that. There's likely also a need for a carb rebuild, but that carb is being thrown out soon enough.

I'm doing a frame-off in the summer and will have everything sorted out by then.