That sure is a low rear end.
Per the master parts list, for 1955-56 PowerFlite:
Tire
size
Ratio Part
Number/Pinion Teeth
6.70 x 15
3.90 1559 397-
20
7.10 x 15
“
“
7.60 x 15
“
1559 396- 19
8.20 x 15
“
“
8.90 x 15
“
1559 393- 18
--Roger van Hoy
Sent: Friday, January 29, 2016 1:55 PM
Subject: [FWDLK] speedometer gear