Here is a pic of my 1960 GMC K1000. The tubular crossmember was not used by GMC to mount the motor. As you know and can see, they used a mount just aft of it. Then the trans was mounted to the next crossmember. My trans crossmember was bolted in and was removed in the pic.
As far as a SBC swap, I would use an aftermarket universal tubular crossmember for the motor. Don't know what trans you are using. The perch idea is for 2WD where there is a place the lower part of perch to land/bolt on the 2WD suspension/crossmember.
This or equivalent may work. There are trans mounts that are the same idea depending if your existing trans member can be used/modified, or not. There are lots of odds and ends to consider.
Depending on your fab skills and where the V8 block motor mounts end up, you can attach other aftermarket motor mounts to the frame or to the existing V6 mounts, or build your own. Best idea for this is to get the motor mocked up in there and think, measure and plan.
Check your front upper shock mounts, they are always broken, cracked or repaired. I have a good method to replace.
All in all a fairly easy motor swap. I kept the V6.
