if your old shaft is good, you could have it retubed to size and rebalanced. should be cheaper than a new one.
the ford pinion will be lower, but i think its a longer pinion also. meaning it will be slightly closer to the tcase, front to back, but slightly lower top to bottom. might not be far off form what you have. if its a long slip shaft it might work as is. im a fan of the 9". light, strong, can go up to 35 or even 40 spline shafts, good clearance, and you can weld anywhere on the housing.
a true hi9 third added to it and it will be sweet. good luck