Justhorsenround/Martin is installing a SBC in a 62 GMC. See his post in this forum. I have a 1960 GMC 4X4. The BB GMC is about 800 pounds wet. A VERY heavy motor, The BBC is not near that heavy. The V6 is mounted, as you know, with rectangular pads on each side and bell housing is supported on feet with tranny cantilevered. Again, as you already know. The mounts and their brackets must ne removed. You can use "perches" or aftermarket Chevy mounts for the BBC. Martin is in the midst of doing that and has good info in his post. It will take a decent amount of work. The torsion bar in 60-62 does not help any.
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Don't worry about the V6. It is well built and will last much longer than any other motor of it's day. It is a boat anchor but I decided to keep it in my 60 because of it's originality and after you read up on it, it grows on you. It's bullet proof. I am a 454 guy all the way but in this case I got a soft spot for this overweight monster. It is a 4000 RPM max motor. If you are cruising at 65 and are happy and at a good RPM, you are good to go.
Otherwise let the V8 fun begin. Whatever you do, GET THE 454 and keep it. The older ones are getting a little hard to find. If the 454 is "aluminized" and has an aluminum bell housing it will be Waaaaaaaaaaaaaaaay lighter than the V6.
GMC used Dana 44 diffs. That will be OK for the BBC. Keep in mind that high cruise speed is all about the trans and rear ratio (including tire size). Hot off the line is all about cubes and trans/rear ratio. Hooking an O/D to a 305 may be a real challenge. I don't know what kind of "performance" you seek.

Don't get all giggly over the word "rebuilt" on that 454. To me that is a red flag unless I know who did it, their experience and if they did it in the dirt or in a clean shop.

If the V6 has good compression, not much smoke, no overheat......it's good to 250,000 +.

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