Hello,
The new truck frame to work with the Hi Adhesion trucks is a great idea. Because you plan on having them socket into the Atlas C628/C630 chassis though, the center axle cannot be centered over the pivot point. The chassis measures 43 feet (via N scale ruler by me) between the pivoting centers, but the actual C636 has a center to center axle distance of 41 feet 10 inches (Source: Win Cuisiniers drawings in RMC, Jan. 1985). It does not seem like much, but it might cause the pivoting trucks to hit the fuel tanks or steps. If the new truck chassis can be made so the center axle is slightly inward, it will make it agree with the prototype. Hopefully that is the plan.
Larry N