Do NOT get a transformer! Giving 400V 60Hz to a motor designed for 400V 50Hz will kill it. A motor designed for 400V 50Hz has the same V/Hz ratio as 480V 60Hz; 8:1. That's what the motor wants to see. If you transform down to 400V but at 60hz, the ratio drops to 6.67:1, about 17% less, so your motors will produce 17% less running torque and 30% less Starting / Accelerating torque. That means they will run at higher slip and pull more current and run hotter, while doing less work. They will prematurely fail.
Add to that the fact that, as others mention, the fans will demand MORE power (173%) at the higher speed, you have a situation where your motor delivers LESS power while the load demands MORE power. Failure is almost immediate.
Feed the motors with 480V 60Hz, then deal with the air flow increase either by using belt drive changes as mentioned, or if there are dampers, closing off the dampers. All you need is for the motor amp draw to be the same as the nameplate rating. However you get there is fine.
You could also just put VFDs on the 3 phase motors, problem solved, budget busted.
As to the single phase motors, you might be screwed no matter what. If the motor nameplates happen to say 230V 50/60Hz, you are golden, but again depending on the TYPE of load. If they too are centrifugal pumps or fans, the same "affinity law" applies; power required by the machine will increase by the cube of the speed change. 120% speed = 1.20 cubed = 173% power. But if you can adjust flow, you can compensate.
but one thing is for sure, you will need to get a 480-230V transformer for those motors and separate out the circuits. 480 phase to neutral is, at best, 277V here. At worst, you have a Delta system so no neutral at all, meaning it is "floating" and potentially there is 480V phase to ground.
Heaters will not care one iota.