Looking from the drivers seat, the low mount starter is where the big hand on the clock hits 4 o'clock. The hi-mount, where the hour hand is at 2 o'clock.
From the engine bay, the low mount starter is below the pan rail. The hi-mount is above the pan rail, beside the frost plugs, very close to the cast iron header.
A 250 is lo-mount, allthe 144-170 andmost 200's generally are hi-mount.
The only issue with using the 200 headers on the 250 is if the starter is small enough to dodge the header outlet. Most cases, its tight, but if you use a #3110 two bolt starter from a 428 FE, a fairly cheap upgrade, you shouldn't have any problem.