The RBT six speed is about an inch longer than a std ZF 5 speed and Panteras have been converted to teh 6 speed so there is that much readily available. You could probably even use the longer offset 6 speed mounting tabs to gain an inch with a std ZF.
The 4 inches Jack talks about is a lot, the Byars big block conversion I saw was nowhere near that amount. Half shaft u-joints won't like turning at that acute an angle so you'll probably be looking at CV axles if you need that amount of rearward movement.
Julian
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