Does the brand of crossover installed make a difference on what part #/size thermostat you should be using? I have a crossover on my engine but don't know the brand, I'm going to put the thermostat and relief valve on it, what I want to know is do I order the original t-stat that would have been on the engine or do i need a different size?

You want a 142 degree T-stat for a big block Chevy. Chances are you will not find it at an auto parts store.