Actually, the early Bachmann Class 46, which I have, needs this same upgrade. Although they put in the good 8-wheel drive system with central can motor, they didn't take the buffer beam off the body and put it on the bogie until a later release of the Peak family.
So I'd like to know how it's done, too!
P.S. I'm thinking that one of the NEM362 series couplers might be easier to fit?
An NEM362 Kadee (#18, #19) might be 'easier', but it's still going to need a hole in the bufferbeam (unless there's a 'trick' I'm missing - hence the posting!).
And you need the Craftsman kit, which I believe is getting hard to obtain. I got mine from the 'spares box' at a Chesterfield model shop last year.
Fingers crossed, someone will respond with some ideas ...