The JB5/JB6 brakes used "low-drag" calipers and "quick take-up" MCs. The heavier-duty JB7/JB8 brakes did not use "low-drag" calipers and were not paired with "quick take-up" MCs.
Mixing parts can yield less than optimum results. See attached.
Is your head spinning yet? Just wait...
The calipers on your D60 will, I expect, NOT be the "low drag" type that requires a "quick take-up" MC, as I assume it will use different calipers entirely from what GM offered on the GMT400 JB5/JB6 brakes (please correct me if I'm mistaken... in fact, if you know what calipers you'll be using on it, do tell). Given my assumptions, you would want an MC from the JB7/JB8, or perhaps JD7/JD8 (see
@HotWheelsBurban's post above), as appropriate for the hydroboost valve body.
AFAIK, the only difference in the HD and non-HD MC is that the HD MC has a slightly smaller bore. There may be other differences, e.g., the non-HD may or may not mate properly with the HB "booster" valve assembly b/c of dimensional differences. Somebody else (
@Schurkey?) might comment.
Bear in mind, if your existing front calipers are "stock JB5", then they are the "low-drag" type which require a "quick take-up" MC... "require" is the operative word here. They'll still work with a non-quick-take-up MC, they just may not work as well (e.g., low-pedal). See attached.
Barring new information to the contrary, I would suggest you install the MC I suggested prior b/c you're planning to install the D60 "in a few months". In the meantime, if you note any braking anomalies (low pedal) with your new MC and "old" JB5 calipers, you can swap JB7/JD7 calipers (which are NOT low-drag and and will work properly with a non-quick-take-up MC) onto the truck in the meantime.