66 Valiant 100 parts swap?
Welcome to FABO :cheers: Got any pics of the Valiant?
Yes, the early 63-66 A-body cars have a different floor pan. How much different? I don't really know. I got my driver side front one replaced, got it from Autobody Specialties, they have all the floor pans, not sure on the trunk though. Also have some fender patch panels.
For the brakes did you convert to the big bolt pattern 73-76 discs or the small bolt pattern 65-71 ones with the 4-piston caliper? I used a disc set-up from a 73 Duster, the upper arms, spindles, and calipers were all swapped over, didn't have to change the lower, it was the same. There is also a company called Scarebird that makes affordable disc conversion kits that use your stock drum spindles and newer parts and still run the SBP wheel. Several member here have used them, my uncle also used them on his 71 Dart and work great.
As far as the k-frame on an early-A go they are ALL the same, slant six or 273, only the motor mounts were different. The 67-up are bigger and would require mods to make it fit, probably not worth it.
Hope this helps!