I just completed an engine - transmission install in my 1965 K10 4X4 fleetside long bed. I did the same thing you are trying to do by replacing the 283 with a 350.
I drove race cars for two decades, so I had my engine builder throw together a little 400 hp 350 engine. My '65 was missing the front crossmember, so we fabricated a crossmember that allowed the engine to mount and the headers to go down through the frame rails. However, my 65 does not have power steering or power brakes and I did not choose to install them.
You'll also need to be aware that the crossmember for the transmission mounts under the bell housing and not the rear transmission mount that is likely to be on your transmission from the '79 donor truck. I used an old toploader 4 speed transmission that was able utilize the front mounted transmission crossmember.
I DID pull the front drive axle out of the '65 and replaced it with a front drive axle from a late 80's K5 Blazer. Had to remount the spring perches and shock mounts, but other than that, it was pretty easy. I can send you photos if you think it might help.
