John,
I'm afraid you might just be S.O.L. There are a number of 6 pdr barrels to replace tank tubes, but the towed guns are so horrible as models that I'm relatively sure no-one has put out a new barrel for them, neither the Italeri US 57mm nor the Tamiya 6 pdr. You might be able to buy a barrel from Resicast, but, if you are going to that trouble, you might as well buy his 6 pdr, which is, by all accounts, stunningly good, if a bit expensive even for a resin kit.
It's a shame, really, as a nice, modern 6 pdr model would be very useful. Lots of stuff to tow it and lots of diorama scenarios to us it in.
Paul