I did a similar conversion years ago but went from 6cyl/C4 to V8/4spdBW and the old tailshaft fitted OK. Some tailshafts have a longer yoke, there's like a couple of inches or so where there is no spline, this was the hassle, I chopped this bit off with a hacksaw on my back step! Very bodgy I know, not recommended! It got me around for a few months albeit with severe vibration until I could get the correct yoke.
You know, somebody actually complimented me on my driving today. They left a little note on the windscreen. It said, 'Parking Fine.'So that was nice.