OP, try not to over think the scenario. Couple of basics I mostly follow, though stated 'mostly'. Meaning not carved in stone and have done and still do trial and error.
That said, I seek the spindle taper 'length' that's appropriate to the crank. They sometimes vary and can disrupt plans.
The classic chain line number is 43.5 mm, measured from dead center bb shell to middle of chain rings on a double OR the center ring on a triple setup. This chain line should follow to the middle of the cog stack. Again, this is a good baseline.
If you're pleased with that Dura Ace crank, use it. I imagine it's properly seated to the existing taper and you only need a shorter spindle. Subtract that difference to get that 43.5. So when you remove the spindle, you should be able to determine the differential length needed. If you can determine the brand and or interchangeable to the existing cups, its a go. If not, you probably can read the cups and note threads size (possibly Italian ... assuming you said old Bottechia) but anyways, get a complete bottom bracket to match threads and the shorter spindle, though make sure its listed to match the taper of that Dura Ace crank. Sutherlands guide will likely have those references.
Last edited by crank_addict; 02-26-19 at 11:19 PM.