1966 valiant v8 swap?

Welcome!! I went thru what you are about to do also. It was interesting to say the least. It seems that the 66 was pretty much a one off kinda car.
If the rest of the car is solid, could be that the seals in the wiper arm shafts are shot. These early A's were notorious for that. These previous owner of mine kept putting caulk and silicone seal on every joint and hole they could find trying to stop it. Can't even guess how many tubes they may have used and it was just the wiper shaft seals.
For the V8 swap, you will need a V8 center link, Schumacher V8 kit or check on here for V8 motor mount brackets.
For the tranny, if you use a 727, you will have to modify the tunnel to accommodate it and have the drive shaft modified. A 904 will work fine and no drive shaft modification, you just have to find one for a V8.
Once again, welcome and have fun!! Dennis