Does the GMC in 58 have a different cross member than the Chevy.
The 58-59 gmc has a different crossmember than both the 55-57 gmc trucks and the 55-59 chevy trucks. Swapping the crossmember with a chevy one will allow you to mount the chevy motor like a chevy.
As for your choice of a 283, not my cup of tea. I built a 59 pontiac 389 for my 59 gmc with 3 deuces. Since my truck was an original v8 truck it was a drop in deal. This weekend I fired up the 302 for my 1940 gmc for the first time ever. It ran perfectly with the 5 deuce manifold. Anybody can put a small block into any vehicle, in fact they do that everyday. The 270s and 302s are tough motors, and have a cool factor that is off the charts.
