first point: if the price / work of changing the kmember is too daunting... you probably should skip this kind of dream project. actually the kmember swap makes it easier for you to fit decent exhaust up to your engine. handles better. strengthens a few things that need it (HDK comes with shock support hoops that tie into the frame, etc). gives you a lot more choices for oil pans. stuff like that.
second point: most of the people who get the swap done (actually finish) have purchased a wrecked truck/car and pulled the whole hemi drive train and control harness and swap it over.
if you are going to do this piecemeal expect a way higher pricetag. and longer build time.
on an old school 360 stroker, 408 with a manual trans or lockup converter you will get comparable gas mileage to the hemi. it's not the motor or EFI that gives the 21mpg. it's the lockup converter or OD. the 727 with old type converter is the gas hog. not the engine or the body of the car.
some people have had good results with FAST EFI also instead of using the factory.
also this wont be done "quickly"
dont pull your 360 right away or your car will be just sittin for a couple of years
