K Member bolt size and best swap options?

The bolt size is 1 1/16". You'll have to decide if you want to keep the same small bolt pattern or to change to the 4 1/2" one. '68-72 has the small pattern and "ice cream sandwich" mounts, '73-6 the large with steel encased mounts. Naturally, if you find a V8 K-member you won't need the mount conversion kit so you can save money there. Other than that, the K-members are just as strong.