85s have vent windows too, unless someone spent a chunk of change and installed after market one piece units. The guts will swap but you might need to drill a few holes. Use the 75 nose and you can stick with round headlights. You will need to swap in the wiper stubs and wiper transmission arms from the 75 into the 85 cab. If you leave the 85 stubs in the hood will clean them off the first time you close it. One of the holes for the wiper stubs lines up on each side. The other two for each side needs to be drilled as they are off about half of the hole. Use the wire harness from the 85 cab as long as it is a two head light set up not a 4 light. Your 75 probably has a mechanical oil pressure gage unless its got idiot lights. You will need to install after market gages, or put the 85 electrical sending unit in.
You will need cab mount bushings for a 75. Is the 75 auto or stick? 2 or 4 wheel drive? Is the 85 cab auto or stick? 2 or 4 wheel drive? Answers to these will determin if the center of the floor needs to be cut out and bolt the 75 center in.
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