i priced out a 727/gearvendor setup against a 518[46rh] setup, and found that one could come out money and modifications ahead by doing a 518.
this included a torque converter, transmission rebuild, beefed up transmission internal components, and a jw ultrabell.
this was with me doing all the work except making up a driveshaft. [which would be roughly the same price no matter which direction a person went]
just my opinion, using my tools and skills. your mileage will vary.