Sorry if this is an obvious question, but I can't find it on Sheldon's site. I'm hoping that it's such an obvious question, they didn't bother answering it.

Can 8-speed Hyperglide cassettes be installed on a 7800 or 7900 hub?

Turning my old Masi Gran Criterium from a wall-hanger into a rider. Going with an Ultegra 6400 Tricolor drive train and building up some nice, speedyish wheels for it.

There were some 7800 hubs that had taller splines that will not allow the installation of anything but a 10 speed cassette. Other than that, there are no restrictions. You could even install an 8 speed cassette on newer wheels designed for 11 speed cassettes as long as you used a spacer.
I used to have a pair of Look pedals just like the ones in your photo. They were a bit heavy, but very durable. I used them for years, then gave them to my son. He may still have them

Go with the 7900. It will work with 8,9,or 10 speed cassettes.
I would imagine the 7800 will be harder and harder to find, because it is a poor design.

Nice Horse!

All 7800 had the tall splines and an Al cassette body.
If you want maximum 7,8,9,10 compatibility make sure to get a 7850 rear hub. They went to a Ti cassette body and back to a normal spline.
No 78xx will fit 11 speed though.

Thanks fellows!
Sorry, I meant 7850's or 7900's. I'd forgotten about the 7800's poor design.
Now if I can just find some 28H 7900's... so far no dice. These used to be plentiful just a year or two ago, but now? Not even seeing much on Ebay!