Maybe this will work. Here's the picture that has my head spinning.
Maybe it will work in terms of space under the floor in a T-3 but I don't think that will work in terms of running well.
1. 48 IDA's on VW motors have lots of fuel reversion. Look at any sedan that runs a decklid and 48 IDA Webers. The inside of the decklid will probably be fuel stained from reversion. It's part of IR intake system. If you give it nowhere to "fog" above the aux vents, you're going to have a hell of a time tuning it. Even the stock single 30-34mm carbs need some room, which is why they run like junk when people remove the stock air filter and put one of those $6.00 chrome n' foam things on instead.
2. asking the intake charge to change direction and taper to such a degree, in a short length, by shortening manifolds enough to stuff those carbs under the floor, I'm not sure what effect that will have.
3. Short air horns creates more of an issue with point # 1
Use DCNF or Dellorto DRLA's. Does the engine move enough air to justify more that either of those carbs can deal with anyway?
Jim