This will depend on equipment your have and your own aesthetic appearance desires. Given the curve radius you are restricted to, a number 6 crossover will be compatible. Note that a crossover is an "S" curve and long equipment (especially passenger cars) will give dramatic overhang. Most modern steam (even articulated) are designed with swivel mounting and can handle negotiation well enough, but the appearance of the ends overhang may be disconcerting. If you are going to build up your crossovers with FastTracks, consider a number 7 or perhaps with number 6 widening the separation distance between the parallel tracks and including a longer straight section between the turnouts at the crossing portion. You may need to modify the trucks and couplers on very long cars to provide additional swing through the 'S' curve.
-ed-
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-ed mccamey-
COSLAR RR -
http://www.coslar.us/NMRA Standards and Conformance Department
PROTO & FINE Scale Coordinator
I estimate I have about 5 pounds of coupler springs somewhere in the vicinity of my workbench.