On using a jack shaft with IDA Webers it is possible. You won't be able the run the existing jackshaft, you have to fabricate 2 brackets at each end, its the jackshaft mount not the actual shaft that gets in the way. I can't seem to find a photo of the final set up we had, this are from the early mock up. We put the Webers on when contemplating racing it but found while its sounded awesome we never quite got to grips with (more like we didn't spent enough time to get to) a proper tune for it. Its was more powerful (or at least easier get the power) with the Holley. Torque and throttle response is much better with Webers though. There are a couple of IDA manifolds which are made and some have the carbs angled which I think could provide even more clearance for the jackshaft.
Hopefully the pictures assist.
IIRC we use chrome moly tube as you need to be conscious of Critical Speed and whip, if going full on to full off with the throttle. but again IIRC we only used bearings at each end not in the middle.
Cheers
Larry
