An brand new external linkage transmission will feel as good as a T56 with 150,000miles on it.

Forget the calculators, savings accounts, install set backs. Spend some seat time in both transmissions before you decide. You will not be dissapointed in the T56.

I stress this over and over again but I'm very picky about shift quality. If you don't really care about quick shifts and just need the overdrive, there are cheaper options than the T56.