Front wheel size help

[ambros]

1965 C10 with stock suspension and disc brake conversion I plan on 3/4 drop I am going to use the grand slam kit from CPP, they are right down the street from me. I would like to put 18x8 boss 338 on the front with maybe 255/50r18 on the front. Rear i will figure out later when we put the bed on.I need to get the suspension on and put it on the ground to do some rocker replacement.The stock rims I took off wont work with disc brakes. I know the disc will push tires out 1 1/8 inch will this work or should i get the kit with original offset. The boss 338 have a 4.5 backspace. Any help would be appreciated.