There is a schematic for using the L297 with a unipolar motor on the L297 application notes pdf (not the datasheet).....
http://www.st.com/stonline/products/literature/an/1734.pdf
its page 15, fig 22.
It involves a darlington array, 4 protection diodes and 4 AND gates.
I'm planning to try it once I get the L297/L298 working together nicely.
Allegro have some new ICs out for controlling unipolar steppers that do the job of the L297 and the darlington array in one chip. I contacted my local sales rep and got some free samples. I haven't tried them yet though. (i'm old fashioned, so I like to learn how to do things the hard way before I use the shortcuts!)
http://www.allegromicro.com/sf/97052/
Regarding heatsinking the L298.... If you plan to step your motor slowly, for instance just a few steps per second, will the IC still need sinking?? How hot does that guy get anyway??