Now knowing that Ron you have a variety of options. You can use the SLOWER ratio early boxes or the much more MODERN faster ratio boxes. It makes a HUGE diff. in driving pleasure. Your box would be a model "808" for the larger cars. Finding something for a newer full size car would be easier than an older box. Starting in '69/'70 the ratios started getting better & by '71 MANY were a faster ratio. You could even use a variable ratio out of a Caddy starting in '65 & MUCH MORE widely available by '67 right up to, I believe, '76. As I recall it's a BOLT-IN swap. Only have to swap pitman arms. Everything else is the same.
One of the ways to tell if it's a faster ratio on one you are thinking about getting. Out of the car it will have 3- 3 1/4 turns, or less, as opposed to 4 turns plus for an earlier box. BEFORE you install it do a re-seal this way you WON'T have to remove the box again. The GM 808 boxes are pretty bullet proof if someone hasn't been messing with it.
Try it, I know you'll like it.
Tom T.