What the original drivetrain would have been would determine in my eyes what a real value might be. Nice thing about 67's is most of the pertinent stuff is listed on the cowl. Without a group 4 on there that Cook mentioned, it's likely a base model RS in which case it could have been a 2 barrel power glide car from the start. Still desirable to own but won't bring the bigger money in my opinion compared to an original 4 barrel stick car, even if the car now has some other form of drivetrain in it.
I've been looking at 67-68 RS's for a while now and being patient I've found 2 different 67's, both were L30/M20 cars so they weren't your basic RS's. Both presentable and rust free SW cars, complete, one ran and one didn't. Both were priced in the upper teens. I almost bought one of them. I've seen nice examples in the mid 20's that were turn key and drive, and almost bought another really super nice 67 RS with it's original 327 2 barrel and power glide, white with blue guts, for $18,500. Again, this one being nicer than the other 2 I mentioned, but a base model RS with a 2 barrel/glide tends to bring the price down a bit.
My opinion it just depends on what the market will bring in your area, but they are out there to be had reasonable if you look around. Really need pics and details for others here to give a more informed idea.
