I can tell you on a bearing insert engine, you can not just change a cap. When I was younger, I broke a Pontiac 455 rear main cap by installing the wrong seal. I pulled a cap from a second block and plastic gauged the bearing and did everything I though was right to finish the assembly. The motor ran about 1 1/2 minutes be locking up. Turned out the rear cap was smaller at the sides then at the top even though it measured correctly. If I would have measured at the deck I would have seen it wasn't going to work. A simple line bore cured it.
Point of my story, measure all the way around ! Second point, find out why it broke.
Pontiac 455 is a different beast, no shims for final adjustment. I believe that these older engines required shims due to less than precise machining capabilities of the time. This comes in handy for making a cap off of another engine work.
