Hi guys, I'm new to the forum, I came here to get some help on just this topic. I have an '85 four-wheel-drive frame I'm putting a '99 5.7 L Vortec into, and I confused myself at first because the motor mounts wouldn't fit, so I thought maybe it was a 454 mounting problem, which is where the cab came from, but it turns out that in 85 GMC never put a big block in that frame so I was back to the drawing board.
It turns out that the motor mounts that were in the frame were broken and collapsed down toward the centerline of the frame, so I replaced the motor mounts on the frame, and the Vortec, with its clamshells from the 99 suburban, fit perfect.
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Just FYI.