With a 904 you should be golden Chief. I just got my Doug's installed on the 66 Hardtop today. It has the 10.5" later a-body style bellhousing and larger clutch. It takes a few extra pieces from Brewer's Performance to covert the car to the larger bellhousing but everything works. I know some other people had issues with the larger manual clutch. But Brewer's is the solution.
Keep your eyes peeled for a future write up on the Doug's install when I get everything totally finished. Putting a new master cylinder and rerouting brake lines to clean thing up a bit. Lots of pictures to come.
I have to say Doug's are hands down easier to install than the tti's. Fitment is about the same. Tti's are a little tighter since they have a larger secondaries. Also, makes a difference if you are running larger torsion bars. Both trap the starter and I would recommend wrapping it with a thermal barrier.
Dave