I think i may have worked out what is going on, i used the couterweights off a s/hand efi 460 which had the hatchet weight but now im thinking the flexplate may have been missmatched ,which may explain why it had the vibration with the weights fitted, it did have a weight on the flexplate but it was very small and doesn't look as big as the weight on the external balance flex plates i have been looking at
does this sound feasable or am i heading down the wrong track again
(p.s. i love my old truck and im happy to put the time in to get it right)
thanks again for all the help

 

Sounds like your on the right track. Drop you pan, verify your crank and go from there. If you have the wrong flexplate. Or the wrong crank spacer or both your going to feel it. Mine shakes but a cam will do that.