Whether you pull the cab off the frame or not, you need to get it aligned to the proper dimensions shown in the Factory Assembly Manual. Get out your tape measure. I think you'll have trouble with the rocker on the right side if you don't pull it off the frame, although some of the guys would disagree. Having it off the frame gives you much better access. I would replace that right side rocker panel with a repop one. Or at least use a portion of a replacement panel. With the condition of your floor, I'd consider buying a
Complete Replacement Floor..
3/16 metal is way too thick. If you're using raw sheet metal, 18 gauge is close to what you want. Supposedly the factory used 19 gauge, which is slightly thinner than 18 gauge, but not enough to make a difference. There are a few items on the cab that are thicker metal, such as the cab support under the floor where that spinning bolt goes through.
As far as vents in the cowl panel, after 1950, the driver side vent door was eliminated, which is the reason your 1953 doesn't have one. The passenger side panel with the louvers on the outside is only used if a Fresh Air heater is installed, with that panel on the inside cowl panel removed to let air into the heater. If you have a recirculating heater, then the panel is in place and the louvers don't serve any purpose.
