Motor mount question 82 4x4 bb to sb

[Rubble]

I got a truck that I am putting together for a buddy.It's a 82 3/4t 4x4. It had a big block in that p.o. removed.I am putting in a 350, my problem/ question is do I need to change out the brackets on the frame as well as the rubber engine mounts? I just swapped the rubber mounts,but it doesn't seem to sit down in the mount far enough to get the long motor mount bolts in.The right hand side is pretty close,but LHS is about 1" out.I measured the frame brackets on the donor truck and they look the same as in the 82.Tranny bolted up good,just won't sit down in mount far enough,any ideas?? T.I.A.
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