Need a proportional valve 75 dodge dart

No, starting in '75 the A-bodies used a cast-iron proportioning (not "proportional") valve basically the same as the one used in the last few years of E-bodies (which Inline Tube does carry, their number VML-105, which would work). Or you could use their PR-100 universal item, or you could use their AJP-01 adjustable prop valve and their PR-104B holdoff valve.