Although a mild chamfer job to line up the
oil holes to the main bearings is OK, enlarging the entire passage between the cam & crank bearings is a bad idea. The most recent 428 in my Fairmont drag car split the block bulkheads between the cam & crank in the #2&4 positions, from the center of the main bearing
oil hole, right down to the cam bearings. I didn`t realize that I had a problem until the
oil pressure started getting lower & lower, eventually dropping down to almost nothing at idle. When I pulled it apart, the 6 year old main & rod bearings still looked great, but the cracks were bad enough to allow the cam bearings at #2&4 to walk out of position, which caused a masive internal oil leak. There is not very much metal around the main bearing feed gallerys, drilling these holes larger would just weaken the block, in my opinion.