question about header size and performance
 

when buying headers what would suit a mildly modded 300hp 305tpi(not yet but well assume so for when i finish with the mods) 1 1/2 with 2 1/2 collectors, or the 1 5/8 with 3inch collectors somebpdy said the 11/2 was better for torque im cluless here

i would go with the 1 5/8 with a 3inch collector.....a 2.5 inch collector just sounds too small...just my $.02...

what is the technical answer here, iknow it has something to do with peak hp and rpm and peak torque stuff like that

For torque the 1 1/2" headers on a 300 hp engine would be best. But I'd still get 1 5/8" headers because they will work better on a more powerfull 350 when that comes along.

but the inch and 5/8 would not give me as much torque on a 305tpi with these following mods listed below, as a 1 1/2 would im im getting this all correct.

Yup.

now how about that collector sizes, 2.5 really make a difference as much as 3inch, my buddy has to build me a custom y pipe anyway so he can fit my other subframe connector in so cant we just upsize the damn collector anyway, talking about a master welder here. or should i look for a 3 inch

Think about it this way: Your exhaust valve is 1.5" inches. Hot exhaust gas likes to expand. Your 1.5" headers are 1.5" outside diameter, and more like 1.375" on the inside.
Now you tell me what you have learned here and which size headers you should get.

so going with inch and a halfs are like the stock size anyway.

Yes , and no. 1 1/2" is about the same as stock , but look where they all dump togather on the stock manifold and what size the stock exit is. 1 1/2" is good for torque,but your TPI is already torquey (spell?). What size exhaust are you running ,2 1/2" or 3"? If your running 3" that would say to me run a 3" collector.
I would go with the 1 5/8" w/3" collectors you will be happier.