I have the cross member modified and the 518 case installed in the car so this is mainly about the drive shaft and drive line mods.

First with the 518 case being 8 inches longer I know it will change the drive line angle so is it better to shim the differential or transmission more to get the correct angles. Either way will be easy right now because the new cross member hasn't been made yet.

If anyone has done this before for a 108" wheelbase car is 42" correct for the drive shaft length after the modification I saw that spec on another site for a A500 swap but it's 1/2" shorter.

Leave the diff as is. Shim the trans to try to get the driveshaft angle as close as possible to where it was before.

I have the A518 installed and it works great everything is functioning as it should except now my speedometer is way off in overdrive. The 904 was working correctly with a 33 tooth gear and the 518 came with a 33 tooth gear but had the electronic adapter instead of a cable. The computer on the B350 van must have adjusted the speedometer gauge so my thinking is there is no way to make a mechanical speedometer work correctly when going into overdrive.

The big question is how to correct the gear so when running on the highway in OD the speedo gauge is correct.

OD has nothing to do with the speedo reading. Just like any other trans its tire size and rearend ratio. The speedo gear is on the output shaft so the trans gearing has no affect.