I currently have the 2.25 edelbrock Y pipe, I was wondering if I should put a 3 inch one on (they sell it seprate) I have a starght pipe in place of the cat which then hooks to the edelbrock 3 in cat back ( it is 2.5 where it bolts tot eh straight pipe/cat could it be adapted and if so how? thanks alot

P.S my mods are below the full exhaust was for the 305 lh4 motor now the car is running a 350.

 

Yes, go 3". For a stock LG-4 they sell the smaller pipe to match up with the little 2-1/4" cat that comes on those cars. For ANYTHING performance-oriented, a full 3" from the y-pipe, to the cat and on back through the rest of the exhaust is the way to go.

You just have a local custom exhaust shop replace your current small diameter straight pipe with a larger 3" unit (or a 3" high flow cat, if you need a cat to pass emissions. You can either have it fully welded or have them flange it for you. They should have a selection of flanges available they can weld in the system for you. Shops that do custom exhaust work don't even break a sweat over stuff like this.

Given you've already got a performance-orented 350 under the hood the difference should be pretty dramatic in your case. Assuming, of course, that you have 3" exhaust the rest of the way back.