it should work, your idea is basically correct.

As it turns out the load 4 - 6v6 see is in the range of what 2-6L6 wants to see.

Its never that simple, but it will work. If you see enough amps, you'll note that there isn't a set impedance. Different manufactures use different plate voltages and different impedance OP transformers. So as long as you are close. That being said 2-6L6GB will want to put out more than 20 watts in most cases. You should go with a 6L6B or a 5881, both lower output 6L6 types. Also, the Sams should have the exact impedance of the 142 op transformer, and the Sams from the one you are working on will have the impedance of what was originally in there. In the "parts listing" part of the Sams the transformers show what impedance they are, and what secondaries they have.

I just wrote in the post below this about OP transformers.

The only thing an ohmeter is good for on these is identifying unknown transformers primaries/secondaries/center taps. The continuity setting helps you find dead shorts as well.

Any actual ohm reading is basically irrelevant. Companies don't wind transformers the exact same way so ohm readings won't translate.

The transformer is windings designed as ratios, not resistance.

The 5k of the transformer is the impedance the tubes want to see, based on the "ratio" of windings needed to have a 4/8/16 ohm output tap.

There's a formula where you put voltage(1v for example) through the transformer and measure it on the secondary, do the math, and you know the ratio and thus the load the tube will see. You can also use this way to identify what ohms the secondaries are if they are unknown.