I ran into similar issues with my '61 K10 when I started restoring the driveline a few years ago. I had the original Eaton rear and closed knuckle Dana/Spicer 44 in the front. I ended up just replacing them with 10-bolt corporate axles out of an '88 K10 Suburban with 3.55 gears. As others have mentioned, you get a tighter turning radius and disc brakes in the front with that type of conversion. The axles themselves are the same tube diameter and hub-to-hub width as the originals. However, you will need to cut the spring perches off the rear and weld on new ones at the correct width and angle for your truck. I mainly made the swap for the disc brakes, but the taller gears should help with drivability. I'm not too worried about acceleration with the SM420 and T221 still intact. I went with the 10-bolt axles due to their availability and lower cost, but you have other choices, like the Dana60/14-bolt full floating combo, if you're concerned about durability and strength. Of course, your milage may vary.
Regards,
Matt
