EMPI is currently "reproducing" 48 ida's (and 51's). I'm quite sure they are being cast in China, machined and assembled here.
They are very close in appearances but I honestly don't know if original parts interchange or not.
With 51mm bores, they are 48's bored out to 51 and those I have seen and the throats are very thin.
51's are mostly VW "Dune Buggy" applications but "technically" a 351c should be a 51mm throttle bore, not a 48mm.
Having said that, i.e., "technically what should work", doesn't always work out?
I would point out that Ford had a few sets of 58mm IDA's made for the GT40 program with the 427 "lightweight" engines. (1965ish)
Neither of the two American teams, Shelby-American or Holman-Moody ran them in competition going to an 850cfm Holley. So apparently they couldn't get them to "work", and if those guys couldn't, I'm a mere mortal, I wouldn't expect to be able to either.
Incidentally. Those 58mm IDA's came in a mahogany case, serial numbered. It's "thought" that no more then a dozen sets were made but that is all "inner sanctum" stuff and was WAY under the table for mortals.