[Bobboloo]

Small spigot 7R cases were the 73' CIS, and some late 73' E and S motors as well. Aluminum cases were 65' thru 67'. In 68' they went to mag. Piston squirters were first installed in 71'.

I think the small spigot 7R case would be ideal for your application but any of the 72'/73' 4R/5R cases would be fine too. Preferably from a T motor as they tend to survive better and require less work.

As for making free reving motors. You can also accomplish this by other means than just a short stroke. Lightweight forged pistons,chrome-moly rods and lightened flywheel/pressure plate are some possibilities.