What year system are they out-of? '54 & '55 were King-Seeley 6-V, and '56 was King-Seeley 12-V; none of these is at all compatible with earlier or later systems! They use a "different" hot-wire, points, and bi-metal sensor which you CANNOT test the same way as other resistant-dependent systems!!!
The very best way to test them - IF they are any of the above type - is to substitute them into a known-good complete system: Resistance measurement alone WILL NOT tell you what you want to know. Resistance-only measurement will only tell you a specific set resistance of the internal hot-coil just until the points open, then you will read "open" until it closes again, at which time it will again read only the fixed coil resistance.
Search back for some of my previous posts on this very subject for details. Otherwise, you will probably throw-away perfectly good senders due to misunderstanding how they are designed to work.
Regards, JLB
55 Ford Crown Victoria Steel Top