Dog,again,very nice work.It looks like we are dealing with similar skill levels,which is pretty cool,and really helps with the advice I am receiving,and any you may be able to offer.
I had to laugh a bit about the 5.5 number when you mentioned it,because we are both working from opposite ends of our trucks.I don't have to reference the rear of my truck at this point because from the cab rearward,my body is going to be a service/utility body with tool boxes,that will be a fabrication.My 2006 chassis is also an extended cab,which was picked because of the extra 2-3 feet or so I will behind the '53 cab to transition from a stock modern service body to the much narrower cab on the '53.
My chassis is a 3500 series with 2WD and A-frame suspension,so my front spindle centerline is what I am tied to.Looks like your's is a 4WD,are you custom locating your front differential spring perches to center the wheel in the fender?I guess at some point I may have to ask you for a measurement or dimension of something in the front but I have a case of brain freeze right now,lol.
The other factor I have to consider is cab mounting height.Is your cab sitting at stock height of the '74 chassis?How is it clearing the transmission? My cab '53 is in very good condition,except for the floor,which I will remove and replace.I am wondering if I should build in some hump in the floor.I want my cab to sit at a low(stock) height.Of couse,I need to verify everything on my own chassis,however,the chassis I will use is on a running truck,used daily.The 2006 chassis humps up immediately behind the cab.