4 bolt mains

Don't waste your time with 4 bolt caps on a production block. It won't do anything except lighten your wallet.

If you need more than that, buy a better block. All the little tricks and gadgets like girdles and all that don't do squat.

Those blocks are inherently weak in the main webs. Putting another hole out there and then pulling on it just compounds the problems.

If you want to verify this, start bolting stuff on the block and watch it pull the main line out of round. Then you can finish the main line and get it nuts on. Untorque the caps and torque them again and watch them move. They will constantly move around.