Your interferance probably isn't at the firewall, but probably with header clearance at your power steering box. If you look, the 69's low power engines used a shorter mount also. This can also lead to balancer clearance issues if you use larger than a 6" balancer.
69 Camaro -originally a LM1 car. 327, Edelbrock E-streets, hooker, DUI performance distributors, ultradyne cam, stewart, TRW etc. Vintage Air, AGR steering, Corbeau GTSII seats, 700R4 transmission, 12-bolt w/Eaton 4.11:1