If they went with a tier swap, I'd support a T70 for SU76 swap over the halftrack. The halftrack feels more synergistic with the SU76 and T34, while having the T70 spot and clear infantry/light vehicles for the SU85 and Rocket Trucks feels more complementary.
+1 this is a fantastic idea.
The way I see it tier three is best combined with tier two and a more defensive army with set up teams while tier four synergizes better with tier one and specialist infantry. The "SU-76 Specialist" bulletin reads "the open fighting position made the tank vulnerable to grenades or mortars, but allowed the crew to better communicate with nearby infantry units."
The M5 halftrack M17 quad-mounted AA guns already provides a great anti-infantry option while the T34 protects set-up teams and defensive flanks. Swapping the T70 for the SU-76 would give a light AT option and barrage making tier three more flexible while tier four would benefit from the T70 flanking, recon and veterancy ability capturing capabilities.