I used angle iron and 1/4 bolts, which seems to be plenty strong enough. I did offer up the engine mount to the firewall first to make sure the holes weren't too far off before I drilled 1/4.
On reflection, I can't see any reason why the firewall can't be drilled final size at this early stage. (Unless you don't have the engine mount yet, in which case 1/4 holes are probably safer!)
Email me for some photos if you like?
Hugh-thomson at hotmail dot com