Rear prop shaft / driveshaft length

I intend fitting a UK market diesel engine into my 1978 CJ-5 during restoration.
I want to also fit a five speed gearbox (probably a LT77 Range Rover item) so there obviously isn't enough without relocating the Dana 20 further back and thus having shorter rear prop shaft.
I'm NOT having a body or suspension lift nor fitting larger tyres than standard so prop angles will remain original.
What is peoples opinion on sensible maximum I should consider relocating Dana 20 rearwards ? Would the prop accept being shortened, say, 12" ?

Thanks.

Paul Humphries,
Stoke-on-Trent,
Staffs.,
UK.

 

Mine is 14.75" and I consider it nearly too short. Under full droop there is very little stroke left in the splines. When I researched my tranny swap I saw some that were shorter though. BTW, the propshaft operating angles will not remain the same if you shorten the shaft - less distance same offset thus different angles.

Karl

 

Tom Wood's has some good articles on driveshafts. http://www.4xshaft.com/
Look in the tech section at "Geometry 101". This article and your measurements should help you plan and choose what's needed.