fuel line compression fittings on '58 TO-35

I have never heard of an "inverse metric fitting"...I'm not so sure such a thing even exists. I know on the gas model TO-20s & 30's, I have sucessfully replaced the steel fuel line to the gas bowl using steel brake line. There are a couple different diameters, take your line with you if you go to an Auto Parts place to look at the brake lines. The gas lines on TO 20/30's are actually a flare fitting, the sealing part is the flared end of the fuel line, and the machined cone shaped bottom in the gas bowl. The nut on the gas line doesn't actually seal anything, it just pushes down on the flared steel line to force a seal against the bottom of the gas bowl fitting. Those fittings are tough to get a good seal, even if the nut threads-in, it doesn't insure a leak free fit if the fuel line isn't straight and centered going into the fitting. I have pretty good luck jiggling the fuel line while tightening down the nut to center and line-up the flared end in the fitting so it seals-up good.

Maybe your '35 is the same sort of thing.