I have a C4 block under my workbench right now to which cross-bolts have been added. I'm the 3rd owner of the engine; it started life as a solid lifter 330HP 'PI' engine. I bought a set of 427 main caps at a swap meet.
It's a straightforward Bridgeport sort of machining effort. I got a set of spacers with the caps, including a couple of spares and it was still found necessary to make several more in order to be able to properly fit things together. The work was all done in Roush's Prototype Shop and they sized them to 4 decimal places. (I just dug a couple of the extras out of my desk as I was writing this.)
Depending on your intended end use, it might be worthwhile to go to the steel ProGram caps. But since they are intended to fit into a 'vanilla' block you'll then need to machine off the internal 'knobs', and have no need for spacers. But you'll still have the extra webbing.
My own block had very little core shift. I now have it cut .040 oversize. As a general rule of thumb, don't bore more than necessary. Cylinder wall strength is more important than the relatively small displacement increase you get from boring. You may find it worthwhile to talk to someone such as Barry at Survival regarding your plans.
Is your block completely drilled to oil the lifters?
KS