I have the NV4500 5 spd trans. conversion in my 92 W250. I never liked the thin-walled 3"x3" square tubing that came with my conversion for a rear trans. mount. Out of curiosity I removed it the other day for inspection and sure enough it was cracked completly across in line with the bolt holes. I made a heavier gauge 3x3 and welded an angled shim on the bottom to make it sit better on the flat bar that rests on the rubber cushions. This might just be my truck,but thought I would pass the info. along in case someone else has the same set-up.
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