Length is not a problem, as L-series gearboxes were available in short, medium and long versions. The shorty versions are the same length as A-series 63 gearbox and gearshift is in the same place as all 1200 gearboxes.

Ball bearing pilot is nice, but doesn't add any perf. It only spins when the clutch pedal is down. The stock bushing is great and last the life of a clutch if properly installed.

there is actually 5 different lengths for the 71 series that i know of

71A which is the r30 and 720(possibly early navara) stumpy box
71B which is pretty much all the other L series 5 speeds
71C:
short(probably a tiny bit longer than 71a), came in later navaras
medium(an inch or 2 longer than 71B), r31, VL commodore, R32 and possibly non turbo r33
long, S13 S14 silvias

Most of what you mention are not L-series gearboxes.

The easy solution for the clutch is to use a 63A clutch plate. It's the same size as all A series clutches, so yiou can use a standard(or beefed up)A series pressure plate which will mean no problems with fitting the clutch into the bellhousing. The spline in the 63A clutch is (IIRC)1" x 20T which means it will fit the input shaft spline of ANY L series box.

re. clutch plate ... i have a silvia 6speed behind my a series (the one on wiki) and when i got it here i got a clutch mob to make me a plate... i use a-series flywheel and an exedy pressure plate and got the button ceremic clutch simply done with a-series diameter external and upsized the spline to fit the silvia imput shaft... works great and having no issues at all

It's not hard to have an A series flywheel redrilled to take the L series 200mm pressure plate. (If you fullas were in NZ, I could have it done for you for under $100) (Freight would KILL it though)
And TBH, you're pissing into the wind spending good coin on an H/D 180mm clutch plate. The 200mm L series one has 36% more surface area. A few years back I got an off the shelf LD20 P/plate, (with 300lbs clamping force) a 200mm plate, (With the linings bonded as well as rivited) and a thrust bearing and flywheel resurface for $168. I may only be running 10psi, but it has oodles of grab, whilest still being a pleasure to drive on the street. Compare that to what you might spend getting a poos and weese 180mm clutch custom made.
I can't vouch for the clearance inside a 4 speed housing, but the larger clutch fits inside a 60 series 5 speed housing no problems.

Now, with all this talk about the spigot.
The end of the input shaft need to be shortened, and turned down. They are both too long, and the OD of the Lseries input is bigger than the OD of the A series spigot bush.
The kick in the nuts is that there is very little to hold it by to turn it. When I had mine done, it had to have the tapered synchro journal locked into a tapered pocket so that it could be held to turn it.