Swapped 454 in 92 K3500 with rebuilt shortblock

[Ray62406]

and now there seems to be a balance problem. Hi, I have a friend who spun a couple bearings in his 92 and took it to a shop who installed a Jasper longblock. It was returned to him with a vibration around 2500 rpm. I checked all the periphials and mounting bolts and it appears to be a internal balance problem. The truck has a 4 speed in it and all mounting bolts are tight. I removed the serpentine belt and the vibration is not in any of the hang on components. The vibration is there even if truck is in gear with clutch disengaged so that eliminates transmission or pilot bearing. You can get it up to highway speed and shut off ign in gear and vibration is there until it drops below 25 hundred...
Is there a difference in early and later model 454s? Where should I look. i have asked and he swears it did not vibrate before the swap and I believe him. Where should we look...

[1968 Pontiac]

Did you change the harmonic balancer?

BTW, if it's a Mark IV 454, they're basically the same since 1970, but you can't use a harmonic balancer from a 396 or 427 on any 454.

[CD Smalley]

Did you get the driveshaft put back in in out of phase?

[Ray62406]

Same balancer, re-used the old one. Driveshaft isn't the cause as it happens while stationary with clutch disengaged and trans in gear.
I used to be familiar with big blocks in the 60's and 70's but wasn't sure if something had changed in the last two decades. The problem is most likely with the rebuild but we are trying to eliminate any other possible reasons before pulling the motor again...