I think you would need a mini tub for anything with more than a 12 inch section width. You MAY be able to fit something slightly less without it, I am planning to attempt it next time I buy tires for my '67, pretty sure I can make it work. Tires I'm looking at are 11.8 section width (275/L60 bias) but its going to be TIGHT and require alot of backspacing. Not sure if it will actually fit but based on my math its the largest I can go. I've got SS springs too which helps.

Of course I'll be measuring more carefully when I actually get ready to buy them, but IMO 12 is the max. You're trying to fit almost 13, I don't think it will work without some modification.