You won't be able to run it at 2.5A continuously from an A3977, since the chip cannot dissipate
the 5W+ needed.
For that current level single-chip drivers aren't going to cope well enough and a full blown
discrete MOSFET driver like a GeckoDrive is the way to go.
Do you actually need the full 2.5A drive? You can run at a lower current if not (within
reason).
The DRV8825 is one of the better (lower on resistance) stepper driver chips, and if only it
came in a package with a heatsink tab it would be mighty...