I think a 1956 +4 would have 16 inch rims shod with 5.25 x 16 crosssply tyres as original equipment.The 15 inch rims are likely to have been a later fitment. Can you measure the width of the rim? I think they should be 4.5 inches.
Assuming you want to remain with radial ply tyres, then you will probably need a narrower section than 165 x15 but I don't know just how narrow is necessary in order to fit the two spares on the twin spare bracket. Also i am not sure just how available such tyres will be in Kentucky.
There are some 4/4 Series 1 owners on here , and I am sure they will along to advise what size os necessary to fit 2 spare wheels.
Once you have this information you have to make s decision. Are the twin spares more important to you than the compromises to ride and handling that may result from fitting the narrower width (say 145 x 15 or possibly 155 x 15)
