Something around 110mm-ish should work, depending on required chainline, and tapers. I've had a few 49D cranks that seem to have quite a range of taper dimensions: some seemed happier on a Shimano (JIS) axle, while others seem closer to Campag. If you use a Campag taper, you'll need to compensate by increasing the length slightly.
Your best bet is to decide which taper seems the better option for the cranks you have, decide on the chainline you require, make a test-fit with an axle you have, measure the resulting chainline, then use that figure to make an educated guess about the ideal axle.
Sheldon has this diagram of a classic TA bottom bracket, with dimensions and suggested chainline, and the note: "Generally interchangeable with older Stronglight". That might give you a clue:
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