ive heard that putting 3" catback on a 305 sounds like ***. but mines not a regular 305. right now its stock except for the intake carb and hei distributor, but ive ordered a comp cam XE262 bumpstick and vortec heads (L31), just wondering if running 3" all the way back how would it sound. whoops forgot about flowtech shorty headers.

 

When I got my car it had a TBI 305 in it with a Flowmaster 3in. single out cat back and it sounded real good. It couldn't get out of it's own way but it still sounded good.
I'm sure headers and a 3 in Y-pipe would help and would not be too much. However, I would also put a 3in cat on because 305's sometimes sound tinny without a cat. It all depends on the muffler.

 

You'll be fine, the headers will be perfect the cat-back will sound better, that might a bit too much diameter for your motor and lose some low end but you'll gain in top end performance.

 

i dont have a cat right now. i just have the headers into a homemade ypipe into a flowmaster 3 chamber where the cat should be and turn down tips on it.