I always ran AE86 front section with whatever rear section to suit the diff in my KE70.
Where are you seeing the gap? Between yoke flange and gearbox end protecting cover or are you looking at it without the cover? I think being able to see 1cm between the output shaft seal and the yoke flange is fine.
Anyway, AE71 tailshaft isn't split (still a two-piece but not joined by a flange and 4 bolts) like AE86 one so you'll have to switch back to borgy to run it. or cut the flange off the diff end and weld that on. which I don't think you'll do/is worth the effort.