transmission and rear wnd questions

[matto]

i have an 80 k5 blazer with the 350 and a th350 and np 208 trans case. the rear end is a 12 bolt with an open diff. i need to find out the gear ratio for the rear end so can replace the ring and pinion gears. how could i find this out? also what do i need to do to change the torque converter for the th 350. and advice would be helpful

thanks in advance

matt