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I have been unable to find what the intake manifold pressure should be on for these '49 and '50 8's. A test kit manual generally states for most cars it should be in the mid-20's range. Can someone please steer me to the correct information or what I should expect to see?

The intake manifold is not under pressure but rather under vacuum. Engines of this type in good mechanical condition and properly tuned should give 18 - 20 inches of vacuum at warm idle.

FYI - technically, vacuum is pressure.
We refer to gauge pressure (psig) as above atmospheric. Where as absolute pressure (psia) is measured from full vacuum, P=0 psia.
"Vacuum" is fractional atmospheric pressure = 14.7 psia, 29.92 inches Hg, 0 psig, 33' H2O.
Eg, 5 psia = 20 inches of "vacuum" = 10 inches of Hg = -9.7 psig.
It just depends where measured from, i.e. relative measurement.
We use common terms, "vacuum" & "pressure" to conveniently distinguish above/below atmospheric but it's all pressure.

Yup, piston aircraft with controllable pitch props use manifold pressure to measure power settings. About 29-30" is standard at sea level, down to as low as 10" at idle with good cylinder compressions, if turbocharged then somewhere in the 30's to 40" or so for normal full throttle. Just depends on what power setting you're trying to measure as to what numbers you want to see.