I've never done them on an E150, but on similar vintage Fords, it was 3/16" line, inverted double flare, 3/8" -24 nuts, except I forget nut size at the ABS module... larger, but I'd reuse them if corrosion hasn't destroyed them.... usually in better shape due to staying out of road grime.
Does it have independent L/R rear brakes or a single run from the front? If a single run, it might have a junction on the left rear soft hose, and you can get a premade length of line with flares and fittings already on it to replace the right rear going across the axle.
If it's a little longer than stock, do a little creative routing to take up the slack. Autozone among other places, sells the pre-made lines with double flare and 3/8" nuts in various lengths. To clarify it's just a straight piece of line, can be bent by hand to be "close enough" to same routing, or use a bender if looks are important.