Go ahead and buy the valves if your MC doesn't have them, which to me sounds like it doesn't. You could disassemble it and see what's in there.

I've used them on a couple trucks with the MC under the floor even though as you point out technically the MC is slightly higher than the calipers/wheel cylinders.
On a drum brake truck there is no downside to having them that I'm aware of, and as 3B pointed out drum brake systems most often have them to start with.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-