i found this,
T_: My 4-bolt main block can't be line bored because a spun bearing ate out too much metal on the block side of the main bearings. There's talk of converting 2-bolt mains to 4- bolt mains. Do you have to buy the splayed main caps or can you use 4-bolt main caps from a stock chevy (i.e. the ones from my new boat anchor), and have the holes drilled out.
LR: Check out the possibility of having the block welded and then line-bored.
S_: Small block 400's are easier to convert from 2-bolt to 4-bolt because the caps are the same width. This is not true with the other small blocks. In my opinion, you have three worthwhile options: (1) Convert to splayed 4-bolt caps. Spendy, but the right way to do it, considering the machine work involved. (2) Buy a different 4-bolt block. You can't walk around a junkyard without tripping on them. They are thick on the ground, and cheap. (3) Just use a two-bolt block. Install ARP main studs and don't worry about it. Unless you are going to spin over 6000 rpm or produce more than 350 hp, I'd go with the cheapest alternative.