If you change to an open drive rear axle you also have to change to an open drive transmission. Not all that difficult but you end up having to buy a transmission and driveshaft along with the rear axle. You could do that with a later three speed that works with a column shift and a mid 70's Saginaw full syncro 3 speed would be real nice (that is what is in my 48) as they shift real smooth and she can even put it in low while it is still slowly rolling up to a light.
On the six lug some of the newer Chevy Colorados have six on 5.5 lug bolt pattern You have to find the right option which is Z85 to get the closest width though. Here is a link to a gear ratio discussion by the Chevy Colorado brigade on the codes. 3.42 might be a bit high for that truck but it sure would give her a nice highway speed.
http://coloradofans.com/forums/70-te...ification.html
Changing the gears in your rear axle would work too but I hear Patricks is back ordered on them and Classic parts wants around 100 bucks more for the ring and pinion.
Any way you do it you can expect to spend around 1000 before you are done unless you get a real deal on a Colorado rear axle and open drive 3 speed or get a rear axle from someone that already has had the 3.55 gears installed and just swap axle and torque tube.