There are 2 sizes of bolts used on those main bearings. 1/2" for the center and rear bearing caps, and 9/16" for the front one. On my 29 engine I used 55-60 just like a head bolt for the smaller ones and 65-70 on the larger 2. I went ahead and reused the existing bolts and retainers. In my opinion much more than those values and you run a risk of breaking the bolt or worse stripping the block threads. After all it is mild steel in cast iron.
