Since the Volt meter in an M1008 isn't showing numbers, I don't know what the actual voltage is when turning the engine over while the glow plugs are on. But I do know that it shows a little bit higher than the meter (still 24 Volt) in my converted M1009 did at full charge. In both cases, the reading is quite low. In the M1009 the needle would be by the + symbol on the meter when charging if memory serves me.
A 24 Volt vehicle should have 25.4 Volts, or two times 12.7 Volts, when fully charged.
Since the voltage would stay the same whether you have one or both batteries hooked up (in your case), the starter should act about the same. But not for as long with only one battery, as you'd have only half the amperage available.
Not sure what you mean by "proper 24v components". A starter?
But if you have the literature for a 12 Volt conversion handy, why not follow the schematic in there (or look at a regular schematic for a CUCV), and compare it to the wiring you have?
There are a gazilion things I don't understand, but I do know that you normally don't have two 12 Volt alternators powering a 12 Volt system, whether that's a single battery or two in parallel. Yes, dual alternators are optional on some modern pickups, but I have no clue how those systems work.
It sure sounds like you are running 24 Volts from the two alternators into what is now a 12 Volt system (possibly not including the starter). How else could the batteries be so severely overcharged?
Please look at a schematic. And do take a peek at the starter, from below, too. There are probably markings on it somewhere.
I wouldn't care one bit which kind of starter it is at this point if I were you. Either wire the batteries to match the charging system, or convert the charging system to 12 Volt.