need engine help...

[69Elky]

ok my buddy has a 61 corvette motor in his 3/4 ton lwb gmc nd we got the motor together dropped it in now we have been putting it together a week ago it started pulling out a rocker stud(think thats right) well we replaced it with a screw in and then it started doing the same thing to 2 more...



heres the engine specs

283 bored .60 over
flattop pistons
stock fuelie type heads
roller tipped rockers(he wanted em)
comp cam power max some thing like 463 ands 481 lift like 280 duration think thats right
and he put stock valve springs in-.-


now we ran a compression test one 2 had ok comp like 120 then the other had a very low # dun member what it was now could the valve springs not have enough force to shut the valves and compression leaks out? lso once in awhile we get a backfire timing everything else is deadon


sry about any spelling errors etc im tired....would appreiciate if someone would take they're time to answer this....