Motor will fit fine, nothing needs to be done. The output shaft on the 74 will be smaller diameter but that just means it will fit the 518, 635 or the 289 crank arm. If you had a broken gear and went to the dealership and wanted an over the counter gear set that gear would include a small output shaft gear latch for the reason stated, it would fit ALL the crank arms Chrysler produced and the parts department wouldn've have to ask your crank arm part number to make sure you had the correct size output shaft on the gear kit.
You can bench test the motor and see if it's dead on a bench also. That'll narrow down it the problem is with the motor or if the problem is with the switch/wiring/something on the car. Switch clicking and 12 volts to the bulkhead connector is normally a dead short somewhere and it's usually, at least more often than not, insulation off the wires coming from the cylinder to the switch plate and wires are either touching each other or touching the housing somewhere where they exit and connect to the wiring harness and the switch plate. But a bench test would certainly give you the same results, with sparks
https://www.passion4mopars.com/How-To-Bench-Test-A-3-Speed-Mopar-Wiper-Motor-Incl-Leece-NevilleGeneral-Industries-Motors_b_4.htmlJim, the new alarm system is working out just fine, to a degree LOL
