lowering an 1987 c-10 with 20s

ok i have a 1987 chevy c-10 and its at stock ride level and im putting on a djm 3/5 lowering kit with spindels and flip kit and im getting eagle alloy 221 series 20x8 rims and the tire shop recomends 265 55 r 15s falken tires but that would 1 inch taller then stock rim and tire and i want these tires they are cheaper and have a milage warrente on them and im going to be traveling from MO to Indiana every couple months so i do need the mileage 135 bucks a tire or i can go with so toyo proxies st 265 45s 209 bucks a tire but the question i have is with i have a clearance issue going 3 inches down and a 1 or 1 1/2 inch taller wheel and tire combo? any advice would be greatly appreciated