OldIHC

I have a fiber oil pan gasket (not cork!) from Olson’s Gaskets. Can’t find anything for torque specs for an HD213 (1939 D-2 truck). Anybody have proper torque spec values? All I’ve found so far is 10-15 ft-lb for similar L-heads with cork gaskets...

Torque the bolts 12-14 ft. lbs. This is assuming the bolts are 5/15" fine thread. Do yourself a favour and chase the threads with a tap and blow them out before installing the bolts. Hand tighten the bolts on the first pass then do the final torque on the second pass.
Support the pan, do not let it hang on a bolt.
Stick the pan gasket to the pan, not the block, this makes installation so much easier. Use a little silicone (black) sealer in the tight corners. A little Permatex anti seize (silver grade) on the threads of the bolts will help to make the torque values better and equal. This is a good substitute for Detroit Diesel "peanut butter" CB 89 will know this product. Detroit Diesel peanut butter works really well on the door knob of the executive restroom. It promotes a "quick release" and gets instant attention.
If you do not have a torque wrench, you may use two fingers to pull a wrench at about 5-6 inches from the fastener. This should give a safe torque when your fingers start to hurt.

I always chase the threads (5/16"-18) and have replaced pans before, but I've also always had the recommended type of gasket along with the manufacturer's torque specs. In this case, the normal gasket would be cork, but I was supplied with a fiber gasket by Olsons. On top of this, the IHC manual for the D-Series does not list any torque specs for the oil pan gasket as far as I can see.

Nonetheless, your torque recommendations agree with mine as far as a cork gasket is concerned, but a fiber gasket has less thickness and is much harder than a cork gasket. I would think it would be torqued to at least a slightly higher setting than cork?

Cork is old technology and a guaranteed leak. Make the mating surface of the pan flat. Check for flat with a straight something. I like to run a flat file over mating surface of the pan. This will remove any high points. You should have used a hammer and dolly to do the first step in making the mating surface flat.
If you decide to use a silicone sealer, do not use so much that it squeezes out. A thin edge should show after tightening.
Modern engine sealing is an RTV and no gasket materials. Navistar pan sealer has been RTV for 4 decades.
RTV would work on your engine if the mating surface preparation is good.