will a 518 work here...

I'm in the process of trying to close a deal (fingers crossed!) on a very clean 69 New Yorker with a 440 in it. My plan is to use this to replace my 68 Fury as my running around car. I have a customers/buddy 1993 3/4 Van that got totalled out sitting on the back lot. The paperwork from AAMCO(!) who did the tranny 7 months prior to the crash is in the glovebox.
I already know about the SB/BB bolt pattern difference that requires something to be done bellhousing wise, what I was wondering out loud is if the swap would be worth it. Of course we all know a BB C-body ain't gonna get 25 mpg even if its going downhill with a tail wind, but will it make a discernible difference and or worth the effort/headache on a car that would probably log 120 miles a week (my current avg.) or the once or twice a year 500 mile car show trip.