Your drivetrain swap will be a very good one. I can't say whether the 20 or 300 is most desirable, they are both good t-cases. My T18 tranny has granny gear... does yours have the same? It shifts like a truck, and first gear is not synchronized, but it is a very stout transmission. The granny gear (low-1st gear) is better than 4:1. I think it is 6:1?? I would be inclined to keep the T18 and D20 gearing, but that's my opinion.
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