Hi,
I don't know whether you will be monitoring this site or whether you will have moved over to the aircraft forum but I'll give it a shot.
I will be of limited help but it may be of some use to you. I have come into contact with the same aircraft but it was known in Canada as the North Star. A number of years ago (over 10) there was a company which sold resin Rolls Royce engines for the DC-4M through FSM. Sorry, can't remember anything else about them.
I do know that the Heller DC-6 had to have the fuselage shortened both ahead of and behind the wing. Not being able to come up with an aftermarket engine pod, the nearest (and it is reasonbly near) RR engine for this application is that from the Avro Shackleton. The Lanc engines are nowhere near big enough.
There also were two window configurations. The RCAF aircraft had oval fuselage windows....others were the usual rectangle. If you're lucky, your version will be able to use the DC-6 windows instead of filling them in and recutting ovals....very time consuming.
If you are not wedded to the 1/72 concept, there is a vac-form kit of the DC-4M produced by Welsh Models in 1/144th. It is a pretty good representation.
I hope this is of some use to you. If you would like more specific details and see some pictures of the one I built I welcome your interest. My email address is: snobrd13@rogers.com Cheers, Mike