I think there are two different cruiseomatics,one that is like a C-4 and the other is like a FMX. I think yours is like the later C-4 and was only used behind the sixes and small V8s and wont bolt up to a bigblock,even though both have removeable bellhousings and I dont think you can get your trans to bolt to the 390. I am not too familiar with the 64 and earlier auto trans but if your original trans has a castiron case then it can be used behind a 390,all you would have to do is get a bellhousing from a 352 or 390 and correct flywheel for the starter you will be using and bolt it togather. 64 and back starters require a different flywheel than the 65 and up so it all determines what year of motor you get and what starter it has and my recommondation is get a motor and trans from the same car as you will have all the correct parts needed to change to a V8.