I'm converting a vintage Trek 720 hybrid to a singlespeed. It currently has a 112mm square taper BB and triple crank that I want to replace with an Ultegra 6500 crank and single chainring. The BB I have for this crank is a 6500 octalink 109.5mm one, but it seems to have no clearance against the chainstay. Is the simple fix to get a different octalink BB with a longer spindle?? Do they even make one that long?

Any input from someone who has done something similar would be appreciated! thanks

there are 109.5, 112.5 and 118.5 long versions. Cost about $50 to $75+
This very weekend I went through every conceivable configuration on my Soma DC trying to get them to fit. Q factor is way low on the double cranks so they don't play well with wider rear triangles. Arms hit, if wider the chainline was wacked out etc. Best to keep it for a pure roadie.
Your triple version may work if it has a wider Q-factor and you remove everything and place the chainring on the outer mount? A small 1 or 2 mm BB spacer for the drive side may also help you get there if the arms aren't too close to the chainstays.

Thanks for the insight. Do you think I would have better luck using a MTB crank with this frame? The original triple that came on the bike I has chain rings that are fused together, which is why I can't use it for a single up front.

I'd say a lower end like 105 or tiagra or just a different road crank should be fine. That Ultegra one is just built with roadie aero in mind. Mountain may be overkill as a triple crank has a wide q factor.
Simple square taper SS crank would probably work fine with the same BB you have assuming its still smooth and a square taper.

Is the crank hitting, or the chainring? Not that it matters because either way, the wider BB will help.

As for Q factor, I always wanted the tiniest Q factor I could get until I was inadvertently forced to ride something wider. My knees loved it. I am not talking super wide.

I have gone through what you are going through and it was a quagmire of Octalink v1 versus v2 and it was a headache.