Depending on how far along in the assembly process the kit is, there may be *very* little work and not a whole lot of cost to convert it to a plain -6.
There are really two major differences. First, you need a different engine/gear mount for the -6. Second, you have to mount a tailwheel.
On the -6, the engine mount includes the mount for the main gear. It is a bolt-up installation on the firewall, so no fuselage modifications are necessary to fit the -6 engine/gear mount. However, if the fuselage has already been opened up for the -A main gear, you may have some holes to patch.
To mount the tailwheel, you'll need to reinforce the aft fuselage by installing a doubled up bulkhead. Also, you'll need to mount a weldment between a couple of bulkheads back there. Speaking as someone who has already finished an airplane, making the aft fuselage modification to a partially complete fuselage wouldn't be a big deal. Beyond that, you need a few tailwheel parts and you're ready to go.
