Often the reson for eplacing the main caps is that they get so pounded so as to distort in shape...distorted as in egg-shaped. Upon main cap removal, the simple test is: how easily did the main caps pull out of the block? If they came out easy, they are junk. If you had to push, pull and tug, they should be OK.
If the latter is true, you can easily reuse the stock main caps (but they need to be cut down to the same heigth on all four) with a girdle.