I'm pretty sure the bolt pattern on the rear of the main case between a 727 and 518 tail housing are different. I would suggest getting a core 727 case, a core 518 case and do a "cut & paste"... Cut a section out of the 518 bell with two mounting bolt locations, bolt the 727 case to the back of an engine, line up a similar cut, then bolt everything together and have the two pieces TIG'd. I hope I've made sense in how I described what I'm envisioning.
Mikey B