This sounds like a good deal to me !
Assuming your 72 is a small block V8 model - 318 or whatever, just use the peripheral items from your 72 on the 340, meaning all this stuff:
Exhaust manifolds or headers (318 manifolds will be pretty restrictive)
engine mounts (from a 340)
trans mount (slant six or V8)
trans linkage
timing cover, waterpump, pulleys, fan, PS and Alt brackets, reuse the 340 vib dampner
radiator/shroud
If your 72 is a /6 then you need to secure all the above from a 1972 v8 equipped car, 318 or 340, - except for the engine mounts, then you'll need the schumacher conversion mounts, readily available. Trans mounts and crossmembers are all the same whether a 6 or 8 application.
The axle and trans are bolt in, The driveshaft might be different length - check it.
The 340 might have the early style water pump and timing cover, if so, convert it to the later style, ie passenger side water pump outlet vs drivers side (as the old style).
Keep in mind the 273 stuff from the 66 IS valuable - sell it or sell the car with the left over parts to someone who wants the thing. Cha-ching! You just made some money to pay for the conversion.
Good luck!