Looking further into this, I think the easiest thing is just to keep the 5 speed block at the back.
The chainset is 50-40 and the cassette is 14-17-20-22-24. The chainline looks fine in the 50x20, so am thinking of just keeping it like that, which gives me a 65.75" ratio (it's hilly round here).
I could then remove shifters, FD, RD, and then adjust the chain to correct length and tension.
Does anyone see any problem in doing this approach? Thanks!