Yet another tire size question please

[GMJimmy]

Yeah I know, do a search! I did but it's impossible to find what I'm looking for. I have a 68 C10 fleetside short box with a 72 C10 rear end. It's lowered 2-1/2 inches in the front with drop spindles and 3 inches in the back (I think). It's got disc brakes on the front and drums on the back. I'm presently using 15X8 Rally wheels 5X5 with a 4.25 inch back space with 245-60's on the front and 245-60's on the back. I would like to put Ridler 645 wheels on it but don't know what size to go with. This is a driver and I'm thinking of using equal size tires all the way around but I need some ideas and opinions on this. My theory is if I have a flat I can use a spare of the same size on any corner. With different sizes front and back and a posi rear end i'd need two spares? The truck would look a lot better with larger wheels on the back but I'm wondering what most people would do. I'm thinking of 17's or 18's X 8's.
Any suggestions would be appreciated.
Thanks.