I bought my '74 Eldorado with an Ambassador 750 and 4 speed installed, and the original 850/5 speed in boxes needing a rebuild for flaked cylinders and ruined heads (chrome flaked: plugged oil pump: destroyed cam chain tensioner: threw off timing: pitted combustion chambers beyond repair).
The Ambo engine, I was assured by the PO, was good.... When I was rebuilding the 850 I pulled the heads off the 750 to see how they looked (were new) and discovered the cylinders in that one were also missing 1/2 the chrome. The sump was clean so the PPO sold the 750 to the PO as a rebuilt engine.... It ran great but was completely trashed.
So, I guess I respect your optimism, but these guys know what they are talking about; they will flake and cost you a ton of money.
Beautiful bike. You have the hardest part done; the cosmetic stuff.