Check rear suspension bushings. Also I had vibration at lower speed. I was using a 5speed driveshaft with that big deadweight at the trans. I borrowed a Ford Motorsport alumium driveshaft. It left so I ordered one. Expensive, ie more than I paid for my car body but worth it. I assume rear trans mount good and mounted properly. I assume proper alignment and highspeed balanced tires. Lot of fun. Almost as much fun as a MG.
Severl sites list proper trans gear for rearend ratio and tire size. That is easy problem to solve. You and me both know a 2.3 turbo sway is not a taken lightly swap. The swap is easy. Making it right now that is where the skill and fun is.
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