Friartuck

Also try an Art Supply Store, look for Acetate.

Brushes hitting a tarnished spot on the armature? Lightly sand the copper segments on the commutator.

Verify fuel delivery. There was an account of an early sixties Cadillac that had a speed issue similar to this one. After many, many , many months of diagnoses and parts replacement, it was determined to be two small dents in the fuel line. The spacing of those dents and the timing of draw for each stroke the fuel pump, interrupted fuel at the high speed. So verify fuel delivery, especially given a new plastic gas tank. Perhaps the pick up portion of the new tank was not drilled out enough? And is the new tank properly vented? Could this be an ignition problem? Coil getting hot? Original Poster doesn't provide enough info on the vehicle configuration.

One vote for wood, though a bare frame is not the same as a whole car body:

This point can't be emphasized enough. Do the research and acquire the right stuff.

What could you put on a rotisserie? T-Bird, Falcon, Lark......I couldn't resist.

It depends on the trunk rack. I like to use Tee nuts sunken into the wood with machine bolts and flat stock perpendicular to the rack's cross members. I do this to minimize visual exposure when the trunk is opened. Some trunk designs use hardware on the side to secure it to the rack. I'm rebuilding a Packard domed lid trunk that employed angle steel into the side corners with holes for bolting to the trunk rack. I may not use them, looks sort of tacky. The elevator bolts or flat head screw method through the base can work with maybe a precovered piece of luan plywood as a cover over the bolts. Makes it look nice. http://www.secondchancegarage.com/how-to/trunk-pt-2-1.cfm http://www.secondchancegarage.com/how-to/trunk-pt-3-1.cfm

Ronnie makes a very good point that I overlooked, protecting the wire itself. So I'll revise my early post to a 12-15 Amp in line fuse assuming 14 gauge wire is used which would be realistic for a 6 Volt harness.

Bloo said it. The instantaneous in rush current for a horn is large and could blow a normal sized fuse. There are "spring oriented fuses" to address this problem but they are usually used in AC circuits, not 6 Volt DC, so the answer is use a large size fuse, like a 20 Amp or 25 Amp fuse. The in rush current would be say 12-15 amps and settle at 6-8 amps., well below the 20 amp fuse size. As for what package to use, use the black phenolic (plastic) in line style screw together fuse holder, one that uses common size glass fuses. Suggest taking the time to solder the wire to the tip in the fuse holder. Makes for a nicer installation. You could use the blade type fuse and fuse holder but its not period correct. Suppose where it's located and if visible and your desire for appearance.

John Wolf and Company in Ohio: http://antiqueinstrument.com/

https://americanarrowcorp.com/product-category/pilot-lights/

Consider small amounts of lubricant at each of the spring clamps to prevent squeaks.

Sometimes a bow is too far gone and its better to replace the wood bow altogether. This is precisely what a Polol can do. https://www.polyall.com/ Available from Canada. This article should help understand what a Polol can do. Unfortunately Kwik Poly is no longer available, but the Polyall 2000 should provide equal results http://www.secondchancegarage.com/public4/kwikpoly-1.cfm Note the use of Pipettes to dispense in small quantities.

Replace the sockets with two contact version.

Adding diodes will not help. Check you are getting full voltage to the light sockets and check grounds, especially where and how the grounds are to the body. Also how the flasher portion is grounded to the body. Assume nothing. Run separate wire(s) as a diagnostic means to isolate the culprit. Let us know how you corrected the problem.