The new cam and lifters will quickly pay for themselves in fuel savings and fix the problem, unless Monster is right and you missed a valve spring. When you pull the cam out, try a dial caliper both ways, on each lobe. Subtract the minimum reading from the maximum, multiply by 1.5 to the get approximate lift. I don't expect two of the holes to be exactly the same. A old smallblock cam is like having eight different cams in one; with each cylinder programmed and directed to do something a bit different from the rest.
Side note.. 7 years ago we tore down a 283 that everyone in town thought was a strong 327. Not only was it a 283 but had never been opened up and worn completely out in every way. The cam was the worst I'd ever seen come out of anything that wasn't skipping. most lobes rounded off but all still lifting at least some. No telling what kind of timing or had or lift, but it was an outrageous tire spinner. A 67 short bed 3 speed and was geared very low. Doesn't take a lot of lift to achieve bottom end torque but I'll never forget how bad that cam looked and how impressive a burnout it did the day before we took it down. Strange thing, but results not typical. I like a new cam