According to datasheet, you will have to calculate the current limiting resistors so that the current DOES NOT exceed the safe ratings of L298. You will have to put them on the substantial heatsink.
As of Max current, 4A is on the chip basis not per channel. Max repetitive current per channel (80% ON / 20% OFF) is 2.5A
Your motors can draw as much as 6.3A per phase, so, to be on safe side for your L298 0.5 Ohm resistors are needed, as stated in the L297 datasheet.
Also you have to ramp-up your pulse frequencu (ramp-up = gradually increase), as the stepper motor can not start istantly at the desired speed due to it's rotor's moment of inertia.
For "smoother ride" use the Half Step feature of L297
On the other side, microstepping is simulating Sin/Cos waveforms of the current through the motor phases, but it is not adding to the position accuracy.
My recomendation for your "beasts" are Gecko G203V drives (
www.geckodrive.com)