All of the "stock" 55-2 trucks I have worked on had the capillary temperature gauges. 56 through 59 had the electrical gauge and sending unit. That doesn't solve your problem as you need the correct sender for what you currently have. It appears that you have a later head that uses the smaller sending unit. Possibly a 59 or later sender would match up.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine