Great. Thanks for the clarification. /6 is currently sitting on a 'bisket' k-frame. I suppose it'll just stay that way until she grows 2 more cylinders! Then she gets to go 'spoolin'...
Sounds like a plan to me. Bolt your V-8 & trans to the donor K-frame, slide the /6 out the bottom, then slide the new V-8 in. The only challenges I foresee are: How to support the brake and control arm assemblies while this is going on? If you're installing headers with the engine, you may also need to remove the steering column for the k-frame install.
Of course, you know the 72 and earlier cross links and steering boxes are not compatible with the 73 and later. IMO, the easiest way to go is to use the donor car steering linkage and steering box on the donor car K-frame.