hey i just bought a goodwrench 350 with 260 horse should i throw my 600 off my 305 on it or a 750 that ive had sittin round for a while??? i do have a dual plane wieand intake it

 

I think on that low output 350, you will like the 600 cfm better.

 

http://www.4secondsflat.com/Carb_CFM_Calculator.html
Try this

 

Carb calculators give a minimum requirement. Larger carbs give a bit more power and a flatter power curve.

As for mileage, not all carbs are the same. You can use 3 different model 750's and get 3 different performance and mileage results.

 

Try 'em both, see which you like better. A rarely use carbs smaller than 750 CFM on anything 350ci or larger, especially with a dual plane (since any given cylinder only "sees" half the carb). You didn't mention if either of those carbs is a double-pumper. If they're both vacuum secondary it's pretty much impossible to over-carburate.