340 build .30 over

You will be fine. I have a 10.2:1 J headed 340 w mp .284/.484 cam runs just fine on 89-91

Two completely different animals. Your camshaft has a much later intake valve closing event, which causes the cylinder to start building pressure later in the compression stroke, resulting in less cylinder pressure.

His camshaft is specifically ground to maximize cylinder pressure and has a much earlier intake closing event, which results in pressure being built much earlier in the compression stroke for more cylinder pressure. I think he will have trouble on pump swill or be right on the edge.