Scribe diagonals on marked face of fuel return firewall fitting (FRFF) and centre-pop the intersection. Put FRFF in milling vise and centre the pop-mark using a pointed-tip wobbler. Lock the travel in both X & Y axes. Switch from ER25 collet chuck to drill chuck in mill. Centre drill then drill 3mm, 4.5mm, 6mm, 8mm, 8mm & 8.5mm in turn, 13.75mm deep using the fine feed (7 turns of 1.8mm + 23 divisions of 0.05mm). Realise after drilling that I had meant to offset the hole 2.25mm from the centre towards the plain end of the fitting so as to leave a little more spanner room around the AN818-4D nut on the AN816-4D union, but it looks as though there will not be a problem.
Hold 1/8" NPT tap in mill chuck to get it started true, then move workpiece to bench vise and continue tapping with tap wrench and Trefolex. When cutting starts to get really hard, stop and clean it all up with solvent and blow gun. Try the fit of an AN816-4D and it goes in satisfactorily far. Remove, blow clean again, apply Loctite 577 and fit tightly. Check again the angle of the AN822-4D and that is also still OK when tightened up.
Will need to work out carefully the sequence of assembly for the fuel pressure sender and the FRFF before making any holes in the top of the footwell.
Re-string the nylon line on the roof pulley & hook so the ailerons will hang clear of the fuselage. Hang each aileron in turn by the hinge knuckles and both are significantly nose-heavy - could even have had a bit more paint!
Return to working on stainless steel firewall. With a ball-pein hammer, try to get the starboard stainless-steel side piece conforming closely to the critical area on the solid steel former. Take the piece to the fuselage and try fitting it against the LG frame. Looks quite promising, but have not yet shaped it to fit the P-clamp near the top of the lower diagonal frame tube. Take it back to the solid former and secure the SS part to the former using a AN3-3 bolt and temporary plain nut. Hammer it a bit more until seeming to fit OK. Transfer to fuselage and insert (with some effort) an AN3-3 bolt & nut to hold it to the P-clamp on the lower tube. It doesn't look too bad, but then adding another AN3-3 bolt & nut to hold it to the P-clamp on the upper tube. Now the SS part will lie against either the upper or the lower diagonal tube, but not both. Somehow need to shrink or put a kink in the starboard edge so that the upper & lower ends come in closer to the tubes. I guess that putting one or more flutes along the edge might have the desired effect.