What airplane is this, and is it designed for a large twin engine? By re-fabricating the firewall, it will need to be heavily reinforced and pinned as it was previously. The GT-80 is a heavy engine and the airframe has to be able to handle it. When moving the firewall back, you have to match the original thrust angles, (if any), of the original installation. If the original firewall had some right thrust built in, then the engine should be mounted slightly to the right of centerline, (as viewed from the front), to allow the crankshaft to be centered to the cowl. If the original firewall had thrust centerlines on it, then those are a good starting point. Also, the building instructions should get into mounting of the engine.

Jesse