The dual seems safer, although my wife's daily has a single MC and stock drum brakes. As long as the parts are fresh, mechanical drum brakes work fine. I like the feel of the non-vacuum assist brakes in my '79 f100 better than any other power-brake vehicle in the yard. . . . I looked at Rock Auto and Oriley's, no listing for '67 E'line MC. I imagine a newer one may fit, but some MC's have bias built in for front disks.I'm not sure, TBH, but the '61-66 were single circuit, and the '67 is a dual circuit. It's pretty rare to find one at the JY.
"if it ain't broke, don't fix it."