fuel cell idea is a good one. id also ditch the rubber line ASAP. im a dealer for classic tube, and im pretty sure they have an application for you. but having it shipped to me and then to you would negate any savings i could pass on, so id just order direct from them. theyre usually pretty close to the factory bends, and just need some minor fitting.
id also say that if the fuel line is rotted, the brake lines will be marginal as well, even though theyre noty leaking yet.
on the front brake front, scarebird pieves work well,m though i personally believe them to be a bit undersized. granted, i have a saying "theres no such thing as too much brakes, just not enough tire"
ive used the scarebird stuff a few times, and it works. though with them being drum spindles, a floating rotor/dual piston caliper/adapter bracket setup would probably be cheaper/easire to accomplish, though i cannot say for sure having never seen your spindles naked.
as far as road feel: part of the problem is alignment specs. part is the primewall 75 series tores and how much the sidewalls flex. part is abysmally low sprig rates. part is shock valving.
if i remember right, these old fords use a single pigtail 5 i9nch coil in about 10 inch uncompressed heght. you should be able to source 6-700 lb circle track springs pretty easily, which with this front suspension design, should be a comfy ride.
power brakes should be a cheap swap awy from a latermodel donor. i personally prefer GM s10 stuff. incredibly simple to adapt to anything.
also, gear-vendors should be able to do a overdrive if your set on keeping the three speed on the tree 4.11 combo, and towing.
also, take a look into a full sized tranny cooler for the FMX. heat kills tham quick.
lastly, im pretty sure vintage air offers a sanden compressor conversion for the 390, shedding weight, decreasing hp drain, and improving cooling capacity. youll have to adjust the ac thermostat, though, if you convert to 134 (at least on GM)
michael