For some reason the 50550 frames are too long, they're the same length as the footplate and wouldn't fit between the buffer beams.
I tacked the frames together and marked where the excess would need to be trimmed - using the Austerity drawings from MORILL I marked the frames with Tamiya masking tape. I'm having a few 'off' days so my tremor is worse than normal, the ability to reposition the tape if it's not quite right first time.
Once the cuts were made, the frames fitted perfectly between the buffer beams.
The frames were separated and I've marked which half etched holes need drilling out for the hex bolts. Shortening the frames mean repositioning some of the rivers/bolts.
I need to find some suitable cosmetic springs and more hex bolts, this thing has devoured them!