You may have MS frame ties or you may be looking at getting it welded to stock floors or frame rails. That is acceptable, the rule says any weld that affects CM needs to be TIG. MS to CM, CM to MS and CM to CM are all TIGed. Beyond that, rules for construction are pretty much the same until you get to an SFI certified situation. The rule book is pretty detailed in it's requirements.

Also, using ChromeMoly instead of Mild Steel doesn't allow you to use smaller diameter bars. Just reduced wall thickness. Doing a cage instead of a bar allows you to use 1.625 instead of 1.750 diameter.