In the Advance Design era if its a Chevy, or New Design era if its a GMC, only 1/2-ton GM trucks had six-lug wheels.
In the Art Deco years, which precede your truck there may have been six-lug 3/4-tons and starting in 1988 GM again made six-lug 3/4-ton trucks.
But '47 to '55 1st, the period your cab and frame are from, there were no six-lug 3/4-tong GM trucks. Anyone saying otherwise doesn't know what they are talking about. It always annoys me when people guess or speculate about such things. Its not hard to look up details like this one in the original documents.
You can look up lots of these details in the
Restoration Pack by clicking my link and find this out. It may not always list a detail the way you are thinking about it, but for something like the issue at hand it does show models and which wheels could come on them and how those wheels attach.
If you read the page about wheels you'll see that the model 3100 (1/2-ton) came with only one attachment possibility. Six-lugs. And no other truck had that same attachment.
'54 Chevy truck wheels.
Your cab my have come from a 3/4-ton truck, but the designations that matter are the ones that affect what parts you order and being a 1/2-ton or 3/4-ton cab makes little or no difference in cab parts. It makes a huge difference in front and rear axle parts, the differential, the brakes and the rear gears.
While it makes a difference in the transmission if you are trying to restore the truck, you have an open drive SM420. That is enough to identify your transmission for buying parts.
It would take checking the casting for numbers to say the transmission could be original or could not be because most the differences are only in the bolt-on pieces.
The transmissions are both easy to swap and easy to convert. Looking to call the truck a 3/4-ton because it has an open drive line is ignoring how easy those elements are to change.
It is possible the transmission like the cab originated in a 3/4-ton. It really does not matter because GM used that same transmission with different hardware bolted on the output end starting in 1948 and through 1967 in 1/2-, 3/4-, 1-, 1-1/2 and 2-ton trucks.
My point is that it being an open transmission means nothing regarding the size of the truck or the year of the truck. The transmissions can be converted from one type to another and were used by GM in different configurations for 20 years. Basing identification of the truck on the transmission in a case like yours where its clear parts have been brought together from a variety of trucks is not going to give a useful answer.
Someone else said they recognized the rear axle as looking like their '55 2nd or perhaps a later Task Force truck. That would suggest the rear axle came with the bed you have. I'll let someone else answer the question as to whether those gears will fit since I'm not sure the details of those axles.
You can call the truck a 3/4-ton because that matches the paperwork. But with a short box and six-lug wheels it looks and will perform like a 1/2-ton and more important when you buy parts you need 1/2-ton parts.
