It was interesting to hear that SunTour came up with the idea for the Black Lightning. In a thread about the model on the Paceline forum, someone posted that in Japan the black/gold combination is associated with hearses. So perhaps SunTour decided they needed to sell the Sprint components...
I guessed that wider tires were part of the answer. I'm not sure what widths were available for 700c in 1985. The oldest mail order catalogs I have are from the early 90s and there's nothing listed wider than 28 mm.
The shifters are definitely newer than the frame. That style with the rounded tops was introduced for the 1998 model year. Crank and derailleurs could be 1995 and were used through 2008. 1995 was the first year that Campagnolo put the model names on the components but they were small and...
Spacing on 8, 9, 10 speed cassettes are different between Shimano and Campagnolo, so I wouldn't expect shifting to be perfect.
What model Shimano shifters and what era 105 derailleur did you try?
The ST700 has the same rack mounts as the other touring models. For more braking power you could add a drum brake. Sturmey Archer even makes one that also has a dynamo...