Jacks correct in another post that having a parts list is indispensible when owning/restoring these cars. As a parts vendor myself, it's difficult (i.e. frustrating) to understand "you know, that thing-a-ma-jig down by that doo-hickey!" "Professional restoration shops" (obviously restoring their 1st Kaiser), are the worst. They don't mind charging big bucks for a restoration but avoid investing in a parts list. Grrrrr.
Anyway, the filler tube hose is in Group 3.52, part number 200462 for all KF products. My 51 Parts List shows it as 3" with no change in the 52/53 Parts List Suppliment. The one I removed from my 54, however, was original and 5". I went back with the 5". 4" would work OK but (IMHO), 3" doesn't give much on each side of the seam between the 2 parts.