Very nice swap. Did you go with a 1-piece or 2-piece drive shaft?
The drive shaft is a 2 piece. Essentially it is an E30 drive shaft with the first 6 inches cut off and the first 6 inches of an E39 drive shaft welded on. When you look at the two drive shafts there is an area were the diameters are the same so it makes sense to cut and weld there.
I put a lot of thought into a 1 piece and the cost of an aluminun 1 piece is a most the same as welding and rebuilding an old two piece drive shaft.
The problem with 1 piece driveshafts are they are not forgiving if there is any slight variation in angles between the transmission and the diff. If want to do a 1 piece you should really take the car to shop that laser measure the centre of the harmonic balancer, trans output flange, and diff input flange to ensure they are perfectly aligned. If not the engine and trans mounts need to be adjusted. That is the process that real race cars use.