1990 2500 60/40 Not Fitting

[TL73]

I have a 1990 GMC 2500 that had a factory bench. I'm installing a 60/40 that I was told came out of a late 80s to early 90s truck. That's all I know. The drivers seat fit fine (drilled the 2 additional holes). The passenger bench bolted up fine in the 2 outer factory holes but the center mounting riser is way off. The back of the riser is touching the floorboard and the back of the bench is probably 2 inches higher than the driver seat. The front of the riser won't even touch the floor board, it's probably 3 inches too high. Nothing appears to be bent or modified. My question is, are there differences in the trans tunnel part of the floorboard in a 1500 and 2500? What about an extended cab and regular?

Thanks