first thing to do is determine the present ratio .1/2 ton ratios are 3.73 3,92 and 4.27 to 1 .a 49-51 mercury rear end will fit by moving spring pads to top of housing .ratios for mercomatic are 3.31 and 3.54 to 1 . the 3.73 0r 3.54 would make a fine package . the 55 chevy car used a 3.70 in stick trans cars .