Runners, air intake or exhaust?

or other?

I thought maybe the runners. When the TPI was built it was done for a 305, right? Then they threw it on the 350 and called it a day. Correct me if I am wrong.

Weakest link?
1 Exhaust
2 cam (on most cars)
3 The tpi itself (take off, throw in trash, put on HSR)
4 heads

The air intake from air filter to the throttle body is a weak link. The biggest single improvement I made was with the exhaust upgrade. That being from the headers to the tail pipe. Do those two and you will see good improvements. The rest depends on your budget.

The weakest link performance wise are the factory "log" exhaust manifolds in my opinion.

exhaust, air intudction, base, cam. Put a cold air intake on it, de screen your MAF if you have one, siamese your base a few inches in (no need for high flow runners after this) siamese your plenum and radius your runners, knock out the EGR walls, and install a larger cam or at least 1.6 roller rockers. The primary restriction as mentioned is the log manifolds, next is the air box, then the plenum, and the base. I've done these mods on a Camaro and it kept getting faster and faster, started out running 14.2's and ended up running 13.5's with a stock cam and heads still.

in my opinion, new headers and full 3" exhaust, along with grinding out those obstructions in the plenum right behind the throttle body will be the easiest upgrades. also a throttle body coolant bypass. other things get more involved such as head porting or heavily porting the plenum and runners.