I finally got hold of a Bachmann SSA locally, which I've wanted for ages in order to try Nigel Burkin's underframe conversion from the Modern Wagons book.
I followed the book's recipe fairly closely, although I cleaned up the original SSA buffer mouldings (which had a fair bit of flash) and used brake details recovered from the PNA's underframe. Not strictly correct, and I suspect there should be more stuff under there but I can't figure it out from the pictures in the book. To fit the chassis to the body without shortening it, I had to trim off the uprights flush at the end of each wagon, I'm intending to make some replacements from thin plastic strip to try and give the right impression.
More interestingly, I found myself looking at the left over ex-TTA chassis, and the Hornby PCA 3-pack which was my first wagon purchase when I got back into UK outline. The PCAs are attractive but completely wrong for Whitemarsh, and I've been wondering for a while about using the (very crisply moulded) Gloucester suspension components to add some more variety to the PNAs. When I realised that the wheelsets from the SSA had brake disc detail, it was impossible to resist. The body isn't strictly correct but I don't really mind about that - 5mm too short isn't something I'm going to worry about.
I cut up the PCA chassis roughly with clippers, and carefully detached all four suspension units with a fine razor saw. The TTA (PNA) chassis had all the detail below the solebars removed, and I drilled out the pinpoint cups on the suspension parts so they would fit the Bachmann SSA wheelsets (keep drilling carefully until there's around 0.5mm between the moulding and the wheel's face). These were then glued to the cleaned up TTA chassis solebars, trapping the wheelsets in place at the same time. I got it as square as I could - so not desperately square - but I think it'll look OK under some paint.
Additional details are the brake calipers, hand wheels, and some other parts recovered from the PCA and SSA underframes, fitted according to photos as best as I could make out. The prototype doesn't have much detail.
It's very rough and ready but runs quite nicely, to my surprise. I'm looking forward to getting some paint on these (and fitting the coupling chains) in the near future, but ought to do the MKA underframes first really.
[Edit: Had the prototype suspension type wrong - Gloucester not BSC - that'll teach me to try and use big words!]