I didn't settle for a specific angle as such. I just tried to get the gearbox and diff to run as parallel as I could. It's not an issue with the driveshaft failing, but more that if the angles differ too greatly, you will get a vibration in the driveline due to the way uni joints work.
When I first bolted everything in using the trans mount supplied in the kit, I had a pretty big slope on the gearbox as you can see below.
I cut the trans mount up and rewelded it 20mm higher so the gearbox was close to the floor, but not touching. I also has some spacers on the engine mounts to make everything bolt up smoothly. I removed them and did some clearancing to get the bolts to fit again and dropped the engine around 10mm.
Lastly I took 2-3mm off the front diff mount to lift the nose ever so slightly so things are fairly parallel now.
As mine was an auto swap, I didn't have an original bell housing or clutch to use. The shifter is a custom one from American Drivetrain to bring it up int eh factory location. The tailshaft is a different length also. I believe my new one is shorter than the original.
If you have any Q's, fire away.