Do you have this manual?
https://www.greenmachinesurplus.com/cvrtx28dx29repair-instructions-common-items-522-2897-p.asp
I built my Scimitar from a pile of bits. My engine mounts were missing. I was told to make sure the top of the gearbox (yellow, red or blue plate with the filler plug) is level with the sponson (horizontal bit of hull above the tracks) - I did this using a spirit level and making sure the vehicle is on level ground. I did have the shim pack and conical washers so it was very straight forward. The double check is to put the fan bulkhead in and measure the distance from the gearbox pulley to the bulkhead at the top and bottom. If it’s not the same your belt won’t last long.
The engine mounts are also shimmed using basically different thickness washers. I had some laser cut in varying thicknesses. I then borrowed the alignment tools to get it at the right height. The back of the engine (not flywheel end) should be slightly higher than the front to prevent the drive shaft from falling prematurely. The engine went in and out 2 or 3 times to get this right.
I’ve not had to touch it since I fitted it in 2013 and I’ve never lost a fan belt, so I think this method works.