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Current set-up in my 54 with 58 engine is a 6V foot starter , but truck is converted to 12V. In completing my switch to 12V key start I will get a starter from my LAPS, and switch ring gears. One vendor has 168 tooth and 153 tooth units, but no mention of 6 vs 12V. Another vendor has a gear to fit 55-59 trucks, but no mention of no. of teeth. In my notes I have written 139 tooth flywheel, but no further info. What do I order with full confidence it is the correct one?

The stock 6 cylinder 12 volt systems use 168 tooth ring gears.
The stock 6 cylinder 6 volt systems use 139 tooth ring gears.

I don't know what uses a 153 tooth ring gear.

A 153 tooth flywheel/ring gear is for later model engines. Not used on 235 engines.
Fred

Three weeks later, and I have removed the flywheel, purchased another ring gear and 12V starter. The ring gear is too big !! It is a Pioneer No. FKG-168S. The flywheel is 13.06" in diameter---about 1/4" smaller in diameter than uses the 168 tooth gear. Per Bill Hanlon's tech tip, this FW is for a 6V starter. I ordered the ring gear for a 1958, which is the engine block. What do I order for a 13.06" ring gear to accept a 12V starter?
PS-- the casting numbers on the flywheel are GM5, 3704832, and K13

Unfortunately, you're going to need a complete flywheel, not just a ring gear. The inside diameter of the gear, and the machined surface of the flywheel is different for 6V and 12V, as well as the tooth count on the ring gear. Either one has the same bolt spacing for the crankshaft.
Jerry

First, double check your casting number on the existing flywheel. I'd bet the "3704832" is actually 3704837, which corresponds to an original 6v heavy duty flywheel for '42-'53 6 cylinder Passenger, and 1/2 ton heavy duty clutches and the same years for 3/4, 1 Ton and Utility sized trucks. See screenshot below.

I think you have two choices to pursue here. The 168 tooth 12v ring gear is not designed to fit the earlier 6v flywheel and to my knowledge, no one makes/offers a 168 tooth ring gear that fits the smaller 6v flywheel. You can either replace the ring gear on your existing flywheel (139 tooth) and use a 6v starter, which can be done with a 12v system as long as you don't crank on it too much, or replace your 6v flywheel with a 12v flywheel and matching 12v starter.

Hope this might offer some help to answer your question.

Edit note - Jerry got to the post button first.

So I am going to ask a question that may be obvious to some but not me. Would it be possible to put the 6 volt Bendix on the 12 volt starter?

No- - - - -the shaft diameter is different, and the 12V starter drive (not a Bendix) has a spiral drive spline. 6V is straight. 6V starters run just fine on 12V- - - -they just crank the engine a little faster. Keep the engine well tuned so you don't have to crank it for very long.
Jerry

I have 45000 miles on my 6 volt starter in my 12 volt truck.