I'm not totally writing the weight off, but I'm not shaving oz's. As I mentioned, if I need to I'll redo the frame and swap the parts I will. I've gotten info on both sides of the coin and except for the additional weight there nothing anyone has said that is concerning (i.e. safety)
Well, I'm using a Tbird 8.8 in my build as well, I'd rather have piece of mind over broken drivetrains than save 5 or 10 pounds. I know other people plan on running a lot less power, where a lighter rear end will still be more than enough. Saving 5 or 10 pounds here and there really does add up in the end. My build is not a hardcore track day car either so I don't mind if I lose a bit of performance. With your VW drivetrain you definitely don't need to use such a heavy diff, but if you want to go ahead. It's cheap, tons of replacements available, and gearing choices from 2.7 up to 6.1 or so. FWIW I believe the Mark VIII(all aluminum) one I have in the garage is 30-35 pounds, and the iron cased TBird one is 5-10 pounds heavier.
Now that I have it 100% complete (i.e. ready for the specific parts of the frame) I'll take it off the table and get a weight for it so I can see just how it fairs.
I'm still trying to find where anyone other than Moti (with that cage of his) has stated what a typical frame weighs so I have something to reference it from.
There really is no such thing as a typical frame weight. Especially when you're dealing with a half finished frame. All those brackets and mounts add up to a substantial amount. I'm building a +442 with 1.25 tubing, and have completed the frame part and am now working on the brackets/mounts part. I can still easily lift my frame by myself, I'd guess the weight at around 200 pounds. That's with the floor on too. With the tunnel and the rest of the mounts, I'd be happy and surprised if it was under 260, and I'm expecting it to end up close to 300.
How do you plan on attaching the VW engine to a RWD tranny? Which tranny are you using?