The MOST important thing you need to look into is the suspension geometry. Simply taking a mustang II steering rack and throwing it into a custom fabbed suspension does not mean it is a good suspension design.
Any GOOD aftermarket suspension that uses a rack and pinion to replace a gearbox style suspension will be redesigned to work with that particular rack. Chris Alston does that and they will work up a custom rack to suite your track width and suspension points.
The other alternative for you (and the one I will recommend) is to simply replace your arms with new ones and just clean and paint. Much simpler and cost effect to just rebuild your current suspension with upgraded bushings, shocks and stab bars. A stock suspension with a few upgrades will out perform any cheap, badly designed aftermarket system all day, every day.
Mark