There are several options depending on what you want to end up with.
1) From reading old posts here, the late spindle will bolt into the early suspension's ball joints.  The taller late spindle should increase the camber gain on bumps.  I have seen a couple posts saying this works okay but I have not tried it.  The tie rod hole in the late spindle is too large for the early tie rod end.  Using a TRB-65GR bushing from http://www.mustangsteve.com/tierodbushings.html seems to work with some grinding per http://www.fordpinto.com/pinto-faq/adapter-bushings-for-spindle/msg132106/#msg132106. 
2) You said you were looking for a replacement rack, implying the 72 rack is dying.  These are available from the drag racing companies like Flamming River but are pricey.  You could retrofit with a 74 up rack like FordMasterTech did here http://www.fordpinto.com/general-pinto-talk/build-pics-1973-turbo-coupe/msg102503/#msg102503 this requires some cutting and welding.  Unfortunately the photo links do not work any more, but I saved them to a Word file if you are interested (pm me your email address).
3) You could bite the bullet and swap out the crossmember, arms, rack, etc. to the 74 up design for better parts availability and a little extra strength.  Not sure of the difference but Speedway has a crossmember for a 34 Chevy http://www.speedwaymotors.com/Garage-Sale-Speedway-Mustang-II-Crossmember-for-1934-35-Chevy-STD-Frame,7873.html.  Although not as attractive, you could liberate a 74 up crossmember and associated parts from the junk yard.
 
Note the 74 up hub has a little bigger diameter than the early hub which is a problem if you are using the 72 steel wheels.
 
There probably are other options too, have fun with it.