Race Engineering offers main bearing spacers that allow the use of conventional small journal bearings in a medium journal block.
http://www.raceeng.com/p-7689-bearin...27-to-350.aspx
I used a set of these in a 350 block with a 283 crank and they worked fine. Be sure to check the tang notches though, the one for the rear main was slightly off location on my set which required a little hand-fitting to avoid binding the bearing, no big deal but it does need to be checked.