Looking for master cylinder suggestions on somewhat unique setup

I don't think the piston has a locking groove for the pushrod rubber bushing (manual-brakes), but that bushing isn't coming out of the late-1990's MC from a Breeze I used in my 1964 Valiant. I slid in the OE brake rod for test-fit then tried to remove it to add a rubber dust bellows. I had to beat the rod out, tearing up the rubber bushing (had to order another). With the 2-4 adapter plate, the brake pedal wound up in the same position as with the OE single-pot MC (search for post w/ photos). But, who am I to argue with Rick Ehrenberg (or Slant-Six Dan) and an internal groove only helps.

Dr Diff also sells that kit of 1980's Dodge truck MC w/ adapter plate and even offers w/ a smaller bore for an easier pedal for manual brakes. Many sell the same/similar adjustable proportioning valve, some w/ brand logos, though Ehrenberg's price is good.