Marc breings up some good points.
Dry type transformers are not rated for overload like the liquid filled "pigs" on poles. Keep the loading at 80%.
Next be aware that not all transformers may have their primaries and secondaries swapped. Don't do it for transformers under 1kva, they typically have uneven primary to secondary turns ratios to make up for their small wire and voltage drop. Check with the manufacturer before using ANY transformer in this manner to be certain.
Otherwise, in doing voltage drop calculations, you need to consider how balanced the load is, the type of tranformer connection (wye-wye, wye delta, etc), even the type of load (harmonics and power factor). Without checking Marc's numbers they are a good place to start.
Stepping up to 600 volts can sometimes increase the overall cost. Cost it out both ways, realizing you also need to redo the voltage drop calcs for the 600 volt and proportionately smaller current.
