To get discs you can use 73 to 79 I beams,get the complete I beams as there are different kingpins sizes and its easier to change the complete suspension.if you can find a truck with a sway bar get it as the I beams are different then the ones without making a sway bar easier to install. Since you want to put a 390 in and if your 66 does not have a FE get a donor truck that has a FE as the mounts will work on your 66,basicly the front frames of the 65 to 79s are the same and everything will interchange only the rear of the frame is different as in 73 the frames were widened a bit on the F100s and F250s that are two wheedl drives. Jeff