I think I've done this. I bought a drum beam from South Africa off a forum member to replace a 'late beam' that came with the bus. I presume now (that I know) the beam that came on my bus was a cross over one as I bought the 73 on disc, pads, backing plates and calipers and they fit.
Note - I haven't yet done the bolts up on the spindles yet as I don't have the spring washers as heritage is out of stock at the moment.
I definitely have criss over clalipers as they are a lot thinner than the 73 on. Writing this has made me think about the need for the conversion bushes (about £12 I think) but can report back as soon as these washers arrive.
Edit - couple of things I found, the backing plate holes didn't match the spindles so had to drill those and if changing the calipers to the thicker style you will need the 4 caliper bolts and washers to suit. The flexibility hose clips should be the same though so you can re-use those.
The more I think about it the more I think the washers won't take up the gap between the caliper and bolt so those bushes will propably be needed.