Help with tire selection.

[Rfhimes]

Hi guys/gals, sorry if this question has been answered before. I'm brand new to the board and this is my first post. I'm having trouble with the search on the board. I just recently purchased a 1970 C10 short bed pick up, pretty much stock but it does have 6 lug rally wheels, 15X8 size. It now has 235/75/15 tires that are worn and need to be replaced. My plan is soon to go with the 4/6 drop kit and the front disc brake conversion, power booster and front sway bar all offered by Early Classic. I don't want to change the rear axles to convert to 5 lug. Early classic has the 6 lug front rotors. I asked Early Classic to give me a recommendation on tire size they said 245/60/15 for the front and 275/60/15 for the rear. I respect their recommendation but thought I'd ask what all of you thought about this tire selection? I don't want to waste money buying the wrong tires. With my present size tires my speedometer is reading about 10 mph slower than my actual speed. Could that be my tire size?
Thanks for the help and your opinions.