Yes a nice "modern" truck 12 bolt differential. Very good. Does look like a crossmember was cut off. This crossmember is near the rear spring mount. Possible stress problem without that. Frame needs checked for cracks in that area Normal universal tubular aftermarket motor and trans crossmembers/mounts have been used.
The 42 3/4 was available as a "regular" or with HDE (Heavy Duty Equipment). This was designated as BM. So your VIN is 6=Oakland, BM= HDE, 08= August 1942 and 1285 is a sequential build number for the Oakland plant starting at 1001.
HDE gave 17" wheels, larger brakes and heavy duty rear springs. 8 leaf.
Front suspension/axle does look original. If disc and bearing kits are available for 42 3/4, that would be the ticket. Indications are that this will be sourcing a issue.
As said, your rear springs are highly modified. Spring under axle. 2 leaves. This would have a lowering affect. Better ride.
All of the proposed mods to a restomod can be done by you at home. Nothing to be afraid of. I am a retired Mechanical Design Engineer (aerospace) also. Good to see you on here.
