Start trolling around wrecking yards for a Commodore fuel tank with integral EFI pump, and start looking at how it'll fit into the HQ without modding anything except for hold-down straps. This way you avoid external pump and swirl pot mounts. If I were you I'd find a crashed ute or van chassis and just cut it and keep the bit between the rear of cab crossmember and the rear shocks crossmember, this way you can play with making up a mounting arrangement that fits under the van rear floor, and get a filler for the new tank added. I have a circuit here that will allow a Commodore fuel sender to operate a late HQ fuel guage. Believe me, this sounds hard but will save you lots of heartache later. The way I like to see these cars done means mods to the RH inner guard but just use another guard for this. The biggest hassle is how to get the wiring through the firewall.