I currently have a 305 with heddman 1 1/2 headers. I rebuilt a 350 and im looking at peak power around 3000 - 3800rpm.
Should I stick with the 1 1/2 or buy the 1 5/8. I header the 1 5/8 will benefit 3000+ rpm and kill lower flow,, is this true?

 

If you only make power to 3800rpm you probably have some other problems. 1 1/2 to 1 5/8 is a small change and either will be fine on a stock or mild 350/305 engine. You should have no problem making power clear up to 5500+ with 1 1/2 headers.

 

with a STOCK 350 you should reach somewhere between 4400-4800 RPM... depending on the engine... with those headers and a decent breathing system (360* cleaner... carb... upgraded injection system) you should be good at 5200-5500, and with a nice cam... and some head work... you should be good up to 6000+
if you have 1-1/2 headers... I'd keep them... the difference between those and the 1-5/8 is minimal... UNLESS the engine is getting pretty radical or up into the 383-406 ci range