First off, Mega parts is a fine vendor. However, they and most everybody else like Roger Gibson, etc. are re-selling fasteners from amkproducts.com. What AMK sells is often either exactly correct for Mopar, or it is labeled "similar". Plus a full description is given, such as on page 52 of their online .pdf catalog, "1/4-20 7/16 AF .62 OD .29 OAH, B-12829 6 pcs S36 for $3.00". Mega Parts doesn't say what size these nuts are.

Three of these nuts above fit the 1/4" studs on the base of my A body blower motor, and the S36 is code for zinc plated plus dipped in yellow-rainbow dichromate solution. Sometimes you can spot the yellow-rainbow coating on the backside of the old washer, but it is almost always gone from the topside of nuts that were exposed to the atmosphere and the elements for 45 years. In other words, you usually won't see the yellow-rainbow chromate any more, and the nut will look dark gray or rusted. To be precise, my factory nuts are "peri" type, meaning the bottom of the washer has pointy snag places, but this AMK nut does not. But the peri part is invisible once installed.

My A body A/C evaporator box has four studs 5/16-18 that penetrate the firewall. The exactly correct nuts for this are AMK page 53, part B10448, also peri and also S36 or zinc dichromate yellow-rainbow.

In both cases, the blower motor studs and the evap box studs are S2 phosphated.

If you are restoring your own fasteners, you can have have them re-zinc plated, as long as they were originally zinc plated and aren't badly rust pitted. The correct procedure is (1) electro-zinc plate, (2) dip in yellow-rainbow dichromate. This should be cheaper than buying new nuts, as long as you meet the plater's minimum order.