If you are going to upgrade the easiest thing to do is by a aftermarket driveshaft with 1350 U-joints and a 1350 yoke for the rear end.
But in reading your sig I wouldn't mess with it with your car, I don't like greasable U-joints though so those would have to go.
I don't know what kind of 60's your car does, but it seems as though once you get into the 1.4's or real low 1.5's the slip yoke that slides into the tranny starts to twist. I haven't seen one brake or do anything bad, it just makes it a pain in the ass to take the driveshaft out the next time, and the bad thing is you never really know when it did it because it shows no signs that there's an issue until you have to remove it for some reason.
I have had that issue myself and what I did was buy a Spicer 1350 slip yoke (If you spend the money and time to upgrade 1350 is the only way to go IMO) and a 1350 to 3R conversion U-joint. Also the Yoke was longer so I had a buddy of mine cut it down.
If had it to do over again I don't think I would go same route, a buddy of mine local had a local driveshaft place make him a 3" driveshaft with 1350 yokes and used a conversion U-joint in the rear. It was in the neighborhood of $350, but I have like $150 in what I did.
Oh and from what I have read 3R's and 1330's are fairly close in size, probably not much of an upgrade there