The forces acting upon main bearing caps stretch them and twist them. When inspecting the caps for cap-walking it is identified by a burnished look to the mating surface of the caps. To answer your question, cap walk is neither fore aft or side to side, its circular around the cap bolts.
The caps in a 351C should fit very tightly in the block, it should be difficult to remove them even after the bolts are removed. That interference fit in the cap registers is a big part of their stability. The outboard bolts of a four bolt block add to that stability, its much harder for a 4 bolt cap to twist within the register. But the outboard bolts don't add much in the way of clamping force, the inboard bolts do most of the clamping.
Another part of the stability of the 351C lower end is simply the "beefiness" of the caps themselves. They are thick and bulky and more difficult to twist or stretch than a smaller cap would be. Look at the picture of the girdle, you'll realize that besides tying all the caps together to resist twisting, it also acts as a steel strap to help prevent stretch. By resisting cap twisting and stretching the girdle reduces the forces that result in "cap walk".
Denny Wydendorf has used a two bolt block in his drag race Mustang. He did as you suggest, he installed pins in the outboard location of the two bolt caps where the outer bolts would be located in the four bolt main caps. Denny launches at 8800 rpm and shifts at 10,000 rpm.
rock and roll
-G
quote:Is "cap walk" in a two bolt main engine at stressful RPMs a side to side or a fore and aft sliding movement?
Could it be restrained by, say, short hardened .250" dia. dowels fitted into holes drilled into the cap and block in lieu of four bolt caps? They could be positioned where the outer bolts of a four bolt cap would usually go and, being much smaller than a 3/8" bolt, would not compromise the two bolt caps(?)
That works and is about the only way to reinforce a 2-bolt Cleveland main cap, since there isn't enough meat on the outer ends to drill for the extra small bolts. FWIW, my Fontana aluminum race block uses two long 1/2" studs, two angled allen bolts AND two 1/4" dowels in each steel main cap, to harness 800+ bhp. These caps aren't easy to remove even with the crank out!