I have a 64 el camino and want to do a gasser should I just bore out my already owned 305 or go buy a 350 block and just go from there I do also plan to bore the 350 not quite sure to what size though.

 

I wouldn't bother with a 305, go 350 and build a 383 or if you can find a good untouched 400 small block go that route. Remember on a 400 you typically can't go over .030 because of the siamese cylinders, so you need to find one that hasn't been bored. My experience with 400s is that they usually need to be decked to square the bore to the head more often than 350 blocks.