How much axial play should there be in the propshaft (free movement backwards and forwards) ?

Putting my propshaft back in today and i cant get it in, with the splined shaft hard up inside the gearbox i cant get the flange at the other end to mate with the diff flange. Its like the propshaft needs to be about 5mm shorter.

With my old gearbox in, it was very tight but you could just about manage it, but now i have put in the new one and have new mounts which are much harder there is less movement in the whole assembly.

Could it be the the propshaft is too long and this led to failure of bearings in my old gearbox?

With no play in the assembly, the movement of the rear axle could cause axial loads to come down the propshaft and into the gearbox ?

I would expect that there should be some allowance for the splined shaft to slide a little in normal use...

Car is a frankenstein with Nissan engine/box and Austin rear end, so the prop has been made up by someone in the past so i wouldn't be surprised if it is too long..

(tried hard to avoid "double entedre's in the writing of this post )