my brother has a 79 305 and we want to build a 383 are the mains the same size as a 350.

 

If you use a 3.75" crank for the 350 mains then yes it will work...you can find those in a 383 kit for the 350 engine. If you use a 400 (3.750") crank then no, you will have to have some work done to it for it to work. Easiest solution is get a 383 stroker kit for a 2 pc. rear main 350 unless you get a 350 block with a 1 piece rear main.