Nathus, Denny's Driveshafts can build a driveshaft for you and ship it anywhere in the world.
I highly recommend you use them or another online service such as;
http://www.sixstates.com/products/categoriessuppliers.jsp?category=DL
who are actual automotive driveshaft builders.
No offense to the company you have selected but if they don't know what an NP208 slip yoke is, or a GM 10 bolt axle with a 3R u-joint, or how to use a seal to center measurement to lengthen a shaft, they are worthless to you.
Anyone with a lathe can cut the end yokes out of a driveshaft but..
It takes special fixtures to hold an NP208 yoke and 3R u-joint to place it in a press to press the end yokes into a longer piece of tubing, straighten, weld the end yokes to that tubing, and in the machine to spin it up and balance it.
I have a hard time believing an agricultural company in Belgium, that is incapable of lengthening a driveshaft based on a simple seal to center measurement, also possesses these GM specific fixtures.
Do yourself a favor and use an online service familiar with American vehicles.
Measurements;
Truck on level ground.
Measure from the seal on the output of the transfer case, straight back, to the center of the cup on the rear differential yoke.
You have an NP 208 Transfer case and a GM 10 Bolt 8.5 rear differential with a 3R pinion yoke.
That is all the information you need to have a quality driveshaft made by a REAL company.
As for changing out the lift blocks later; it won't matter if you keep the same amount of lift.