Heavy Knocking

Here's the issue:
Its a 58 Olds 88 with a 371 - rochester 4bbl. In park/nuetral the car will rev great with no apparent problems. Once in gear it will run OK for a couple miles. Then as you give it gas it start to knock very heavily, when you let off the gas and come back to idle it runs fine. Sounds like a timing problem under load. The centrifical advance mechanism appears to be fine as does the vacuum advance. Any ideas?

[another old car]

this is just a guess , you didnt say how many miles on car
if its a high mileage engine , you might need a timing chain
is it a knocking sound or spark knock