High spoke count rims and hubs have never been widely available. 40h and 48h rims are still produced by Velocity, and Shimano and a few boutique makers produce tandem hubs. A 36h rear wheel (32 or even 28 will suffice on front) is usually adequate for loaded touring provided the load is not too great, riding is restricted to pavement, or a bit of both. In my experience 36h is good for up to ~250 lbs total, 32h up to 225. If you expect to exceed 250 regularly you should consider tandem grade rear wheel (if you exceed 300 lbs you need custom wheels
and frameset ).
WRT to Ultegra hubs, the narrower OLD/rear spacing of 130mm exacerbates the dishing on rear wheel, leaving the drive side (DS) spokes with relatively low tension compared to the wider MTB OLD/spacing standard of 135mm. For greater rear wheel longevity, you'd be better off building 32h wheels with Deore or XT 135mm hubs, which is why nearly all current touring bikes use 135mm spacing even with "road" 700c wheels. Of course 36h 135mm rear wheel would be even better. Pay attention to the hub flange distance-to-center - the further the DS flange is located from center, the less dish / more tension one can build into DS spokes. A few rim makers (Velocity) make asymmetrically-drilled rims, with the holes offset to the non-drive-side, which helps achieve significantly higher DS spoke tension. Velocity has several rim models for touring, the
Atlas looks to be a good candidate for a long-lived touring wheel. Finally, try to determine rim manufacturer's recommended spoke tension for wheelbuilding. The more tension the better, up to the point where the nipples eventually deform and/or crack the rim's spoke bed as the nipples try to pull through rim. For most rims you should probably limit spoke tension to 120 kg-f. Velocity used to quote 135, but if you exceeded 125 you could expect to have a few cracks a couple thousand miles down the road. Buy, borrow or hire someone with a spoke tensiometer (
Park) for best wheelbuilding results.
Added for post
#6 :
Expensive DT Alpine spokes are overkill and overrated for touring use.
Wheelsmith DB14 (made in the USA) are more than adequate for the purpose, and cost and weigh less than the equivalent DT Competition. I've never broken a WS DB14.
Your biggest mechanical issue will be the narrow rear spacing of the 1983 bike, which will contribute significantly to dish and low DS spoke tension. The beefiest spokes in the world won't help a bit if you can't tension them adequately due to the geometric limitations of the hub (and frame).