Josh Watson, Addison, NY, 1964 Plymouth valiant V200 , 225

I have a 64 Valiant V-200 with the original 225 slant with the push button auto, I'm working on an engine swap, which is a 273 with slight mods which include a mild cam, complete aluminum roller rocker arms, double roller timing chain, performance electronic ignition, 570 cfm carb mounted on a Edelbrock intake and headers. As I'm slowly working on this I believe I'm coming into a problem which I need to resolve fast: the Valiant curently has the small 4" bolt pattern but I already bought the wheels that I really want to put on the car. Do you think I should put the 8-3/4 rear in as well as disc brakes on the front? I'm not working on a huge budget and really don't have time going through the scrap yards looking for parts either. I'm not sure what to do. The car is a huge project in my life it was my fathers and my project car and he passed away just after we started it. I'm only 23 and don't have a whole lot of knowledge, so any help would be awesome.