Thanks for all the responses so far. It does look like I'm in for a conversion job. I'll take a look at the TMD site for the upper hulls, and check out the Asuka Firefly IC and the Asuka M4 "Fay".
One last question - is there a definitive resource/book on Sherman variants? I'm a "returner" to the hobby, I used to have some books "Nuts and Bolts" and a series of Polish books on panzers when I had a preference for German armour - these used to have great fine line drawings of variants etc.
I will post the project once it gets off the ground.
Cheers,
Paul