Any question answered...

How long a Shimano square taper BB will I need to run a 7400 series Dura Ace road chainset single speed (chainring on outside) with a track wheel running SS. Wheel is 120mm spaced to 126mm for use on a road frame.

107mm should be close, assuming it's actually FC-7400 designed for 112mm BB as a double. FC-7410 uses a 103mm BB for double, so you won't be able to bring the outer ring in any further to get a track-ish chain line. If you end up experimenting with different lengths, bear in mind that you might be able to save some money by buying your own BB tools rather than repeatedly taking the bike to shops for BB swaps.

I'm going to put a 38t chainring on my sram crankset, should I drop the big ring down to a 52t or will a 53t work?

Modern road front dérailleurs are mostly happy with up to 16t difference between chain rings, so 38/53 should be fine.