At what point would you suggest Aftermarket Axles.

we have a set of stock axles drilled to 4.5 bolt patt. in a 73 duster all steel, fiberglass bumper and hood scoop,2 race seats,no back seat,8 point cage with ladder bars and coil overs, stock frontend just rebuild, that has been 6.20 in the 1/8 mile and no problems..strange spool and gears.....carries the front tires about 3' in the air and about 2/3' out...451/indy iron -1 heads ,glide,and 5500 verta...stock axles are tough..jmho thanks mike