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Upgrading from downtube to modern shifters
I have a 1987 Cannondale SR500 which I am considering upgrading to a more modern drive train. I am leaning towards collecting Shimano 105 parts from eBay with a triple chain ring. I have various things I need to figure out. For the moment I want to understand how to route the cables beyond the down tube on up to the combination shifter/brake levers on the bars.
Any ideas?

There are very standard cable stops that replace the downtube shifters. The cable housing runs from the "brifters" to the downtube.

A bigger issue is the rear spacing. Is your bike a 6 or 7 speed? Either way, it probably has 126mm spacing in the back. 8 speed and above uses 130mm spacing. It's only 4mm difference, but it's generally not recommended to flex an aluminum frame. Ideally you'd find some old 7 speed parts in good shape, but they're getting rarer.
There's ways to play around and fit the 8-9-10 speed hubs in there, but then you're also buying a wheel or at least a hub.
Triple crank might require a longer BB spindle.
How do you measure this spacing? What is on the bike is a 6 speed SunTour with a Maillard 400CX sealed mechanism 36 hole hub. When I measured the spacing between the inside of the frame drop outs I only get about 107 mm. Since that is so much less than what you suggest, I suspect I am not measuring the right thing.

Actually I found a good picture at that shows how to measure. I will try that and see what I get.

I don't know why I got such a small number before. I clearly get 126 mm. So I can look for a 7 speed freewheel but I imagine I need a threaded one.

Yes, you would need a freewheel and not a cassette. If you get a shimano or "shimano-compatible" freewheel, it will work with shimano 7 spd shifters. You might be able to use an 8 speed shifter with one click locked out. But I'd do a little checking on compatibility between 8 and 7 speed stuff. I can't remember what all works with what.


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