I'm a bit foggy on the year when the column shift dissappeared, If your 71 has column shift you'll need to get your hands on a later model steering column, then it should be a bolt-on deal. If you don't have the column shift, I'd think you could bolt the 79 box right up. This would be a good time to get a new rag-joint, it can help get rid of some steering slop.
I don't think the column shift gear will bolt up to the 79 steering column, why not swap in the 4-speed transmission from the 79, it would be better on the road and it's an easy swap, it sounds like you already have most of the needed parts. Or if you want to keep the 3-speed you could convert it to floor shift, Specter off-road and Man-a-fre sells conversions, also you could try cruiserparts.net for the required parts. Either way, given that you have a 79 parts truck, you have quite a few options.
When I got my cruiser the 79 came with the deal, the body is mostly shot, but the PO said the tranny, axles, and transfer case were all good. I have considered swapping to the 4spd but im not so sure if my mechanical abilities will allow that.
If you want a floor shift for your 3-speed, a got one $10. plus shipping if you want to go that route. If you go with the 4-speed and transfer, it's not that difficult, you can do it in your driveway with a floor jack, or use a 4x4 across the front doors with a come-a-long. I'm not sure about the clutch, but you should change it while your in there.
What condition is the shifter in? I would really like to go that route and change the entire column with the '79 (the steering on it seems to be really tight). What all is involved with installing the floor shifter?