Measure the rear output flange on the t-case to the rear flange on the diff. That will tell ya what length drive shaft you will need like Anthony said.
I just got done doing that with my 82 I just dropped a G54 into it.

I would use the forward shift t-case with the doubler first then use another t-case behind it

I measured both the L-52 with the t-case from the front of the bellhousing to the back flange it is 34 1/2" The G54 Transmission is 38 1/2" long measured the same way.
So add the thickness of the doubler and the extra part of the t-case.

The rear driveshaft I had was hosed anyways it was bent and twisted. Anyways I am getting mine shortened resleeved and balanced with new joints for about $250.

I measured both the front driveshafts

The 79-83 driveshaft's are 19 1/2" compressed and 23 1/2" extended and still has 2" of spline left
and can move upto a 35 deg angle

The 86-95 IFS front driveshaft's are 23 1/2" compressed and 28" extended with 2" of spline left.

For the front I am using the IFS front driveshaft to replace the old front driveshaft it should work decent once I modify the cardigan joint to go 35 deg.

http://www.4x4wire.com/toyota/tech/cvmod/