JonW has trodden he 421 path and documented it on his website if I remember.
Interesting point, its a bit like the cost of a resto, just talking to someone flogging a rustbucket RZ today and refreshingly he was very realistic about the REAL cost of a resto, I suppose its the same concept here, it doesn't necessarily make sense, but then maybe making sense is missing the point.
On the other hand, the 500's are getting bloody expensive, and if you buy one that needs the motor reworking you're anecdotally in for a seriously stiff bill.
Ex UK, now in Adelaide. LC250/350. DT175. Shed full of sh1t in the vague form of dismantled rusty RD’s and RZ’s.
Nah you can build a 421 for way less than you can restore an RZ500... As BRG says; See my site for my thoughts on prices compared to upping your 250 to 350 compared to a 421 etc. Plus my work on doing it. Enjoy a 421... I do.