Hi
Just did this on 57 GMC, also have 56 3100 with 283. Both of mine use automatics so a rear automatic crossmember was installed. I found it easier to have the engine and transmission bolted up in order to position the assembly. Using the automatic transmission the front to back location of the engine is dependent on distributor to fire wall distance. Height of the front of the engine and the angle of the carb mounting flange were set up by height of the crossmember mount off of bottom of chassis web. I did not weld the caps in, makes it difficult to pull the crossmember, with crossmembers bolted to chassis and engine mounts in place and engine centered I drilled one hole through cap and tube at each end and used grade 8 3/8" bolt to lock it in place.
Original steering column cleared rams horns on the 283 but have since switched to PS with tilt column on the 56.
Hope this helps
Vic