The brass block on a '69 is likely just a distributor and warning switch. GM as a rule did not change to combo valves until '71. Before that, they had a separate hold-off (metering) valve for the front, and many models also got a separate proportioning valve for the rear.
You can easily tell if it is a combo valve or just the dist/switch. If it has a rubber button and a big nut, it is a combo valve. Otherwise you need the separate valving for a balanced system.
The 1/4" master line goes to the rear brakes. It is 1/4" from the master to the switch/valve, and to the rear hose, where it Ts off to 3/16" across the rear axle.
The 3/16" master line(s) go to the front. Single 3/16" line from master, then two 3/16"lines coming from switch.. one to each front wheel.
If the master has resivoirs of the same size, GM usually put the front brakes on the front resivoir back then, but it could be either way. If one resivoir is larger, it goes to the front.
You may be able to tell by the brass seat inside the master ports. One might have a larger flare, and goes to the rear. You cant tell by the fitting size, as the line fittings were all different sizes for ease of assembly. If you have the OE lines and master, you may be able to tell from that because the fittings do not interchange from port to port. They may also be the same fitting size for the master and valve on the same circuit though. (hope that made sense)
Yes you need to bench bleed... Always.