[Synchro909]

I have one car with both the MItchell transmission and O/D. We do lots of miles towing a camper behind that one. I have another 3 cars and all of those have the original cog house and a Mitchell O/D (gear splitter). One of those also has a 3.54 rear end (1928 Tudor) with a B engine fitted with a downdraught carby. That car doesn't handle hills all that well but I can sit on 60 mph easily enough in it on the flat. I think I prefer the standard rear end with O/D. The original gearbox is fine.
In your case, I'd go the O/D