1 - 15's are actually correct for yours, tho' 16's were optional for higher GVW rating - the only thing you'll find w/ the hubcap clips already on is 55-59, tho the later wheels can have the clips added [I think they were drilled there till the 70's] - lots of later truck 6bolt wheels fit, and will take 'period correct' caps - I've got later 15's on my 58 3200
2 - get tires at yer local dealer rather than the spendy things sold for 'vintage vehicles' - I got some Yokohama 7.00x15LT radials and love'em, tho being 10ply they ride a little hard w/ no load - good profile and footprint appropriate for the 50's as well as being safe - yes, you'll regret bias plys, and there's no reason to use'em unless you're into show quality resto
3 - 6.5x16? ya mean 6.50? ya won't find 6.50 other than in the 'vintage' bias ply, and the driving you're doing, the radials are much better, tho the car size wide footprint are hard on the steering gear [and harder to steer, especially as they age], so look for narrower, which the trucks were designed for and actually are safer on the wet and snow
the 700x15's I found are the closest thing to the original height and width, but only available in an LT and stiff - in the end it's what feels good to you, but I like that these tires feel like power steering and will prolly last longer than someone my age

others are more interested in a car-like ride
Bill