You could also try something else - and have a relatively cheap "quick change" setup by going for a bigger 15T front sprocket, with a 43T rear - which gives you similar gearing to a 14T/40T combo, and then having the option to switch to a 14T front, for an even shorter gearing for the very tight stuff.

Or, go for a 15T front with 42T rear, which is similar to a 14T/39T combo, and a nice alternative shorter gearing by switching the front to a 14T

....just a thought!

BTW - I think all stock 748's came with a 520 chain, unlike their bigger brothers....