SU-76M - Site Main Gun: Locks down the unit (immobile, but can rotate in place); boosts le sight range, boosts le damage, reduces le reload time, boosts le penetration (not by ridiculous amounts, mind you), like, oh, IDK, the Marder--aka, the SU-76M when it was actually useful
Also consider Rapid Fire (a-la the Geschutzwagen) or adding a pintle mounted DT LMG (similar to the MG42 on the Geschutz)--I don't know if the SU-76M was equipped with some way to mount a pintle MG, but it couldn't hurt.
Some sort of suppressive fire ability to the T-34/76 that increases the burst duration/accuracy/burst duration+accuracy of its hull and co-axial DT machineguns, but increases the reload time of the main gun (nothing huge, maybe a 10% penalty for the main gun)? Something to make it as effective in the AI department as the Pz IV/Sherman.