I have a K-member that I pulled out from under a 65 valiant wagon about a decade ago. Last night was the first time I ever really looked at it real close. Here's my problem it has two c-channel pads where the motor mounts attach. I always thought all k's from those years were the same for 6 and 8 cars. I didn't have a chance to check my dart (my brother borrowed it) but both of my valiants do not have pads. On the valiants the mount attaches directly to K member. It is definately a 66 and older k since it have the old style idler. Anyone have any guesses. Another odd thing about the wagon I pulled it out of it had a barracuda interior with rear seat that had the fish on it, half a driveshaft loop, rear mounted batt, and 8.75 housing sbp axles but no diff, and a 66 console with floor shift. I kept the parts I mentioned but the car was scrapped long ago. Thanks in advance.

Given all the other high-perf mods you mention the donor car having, the most likely explanation is that the previous owner modified the K-member for some reason or another (reinforcement, installation of a nonstandard engine, etc.)

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You're correct in that the same K Frame was used in both 6's and V8's for 64-66. Notor mounts were obviously different. Don't know about your particular frame but perhaps it had something to do with drag racing. You mentioned pulling a driveshaft loop which indicates to me it might have been used for racing. Brrracuda interior in the wagon - sure why not? Easily swapped. I'm doing the same thing to my 65 Dart wagon - swapping in a 65 Barracuda interior mainly because Dart 270 interiors are not being repro'd and Barracuda interiors are. My original seats are completely shot, frames and all, so it was a no brainer to me.