As for d/s and yoke - I did exactly what Nachodog mentioned - took it to a driveline shop and had them cut it and install a T5 27 spline slip yoke. Had new u-joints installed at the same time. Cost around $100. Some cut em at home, but NOT me...
Doing it this way - the rear part of the driveshaft remains the same, just a stock 66 u-joint connecting to the stock pinion yoke, just like it already was.
bigyellow - excellent question and I wish I had an answer! Unfortunately, I just got my speedo working about a month or so ago. I've been chasing my tail on an electrical gremlin since then (unrelated issue - has nothing to do with the trans).
So, I can't really tell ya how much better it is. Technically, I should be using 70% of the gas I used before if all I did was drive around in 5th gear - but to be honest, around town I seldom see 5th, so probably not much improvement realized yet. BUT, the fun factor is way high - and it should do a good bit better on the highway.
EDIT - when talking to the driveline shop, make sure to remember to emphasize to them you're working on an S10 T5... I was talking "S10", but he was listening "Mustang" - kept trying to sell me a 28 spline slip yoke instead of a 27. Mustang hot rodders probably send the driveline shop more biz than we do when it comes to T5 upgrades.