Webers on a 928 engine have been done. There was a street rod done in California in the 80's. More of a show car than a driver.
928 engines are a very different animal. The US spec engines are only 218-219 hp. Kind of a waste of time when you can bolt in a 6 with no drama and get that easy.
The euro engines stock are 300-310 advertised hp.
The engines are higher compression, have larger valves, bigger ports, the inside of the spyder runners are larger and the throttle body is way bigger than the US engine.
There are very few mods that can be done. The challenge is the block. It's made of allusil. The whole block. You can only run the special iron coated pistons. Try to use aftermarket pistons and they stick to the block.
The other limitation are the cams. They are a long duration short lift designed for F.I. Not suited for carbs.
The engine I have is what is called a hybrid. I am running a 5.0 block and pistons. The 5.0 is a 32 valve engine. I had the piston notched for 16 valve heads. I have a complete euro-spec "S" 16 valve top end with CIS. This combo is producing over 350 RW hp. in the 928's turning all the accessories. I'm only turning an alternator.
Oh: and I used a 901 with an HB 5th and sorted stack. It's a custom length side shift. (as in I cut the housing with a router to make it fit. No one makes any conversion parts. I made my own adaptor plate and custom flywheel and clutch.
I drive the car hard. Sandy and I have been to almost every corner of the country more than once. I built a cruiser not a town car.