So what did ford change for them? - Injectors, MAF?

Nigel

Nope they still ran 19's/55mm so nothing to change.

Cool. Mine runs great on LPG, but s**t on Petrol.

Looks like (According to AFR's) its going hard-lean as soon as theres throttle. Fuel Pressure dropping, or Fuel pump not pumping. Or the MAF - but the MAF was fine. no backfires or codes either

Yeah it's not often you'll get a code for MAF because it usually needs to be severe for the EEC to "know" its faulty. If it's putting out a reduced(offset) voltage the EEC has no real way of knowing this unless it picks up a short to ground or similar.
Pump is a likely bet.
I take it you cut injector power when on LPG?

I do at the moment - Im hoping to move to vapour injection though so its not a permy change. Cut injector power, run additional power to the ISC so that still works.

Actually the 5.0's are a nice easy install. I also took the liberty of cutting the o2 sensors on LPG so the ECU doesnt miss-learn. Normal installs seem to cut one, but that leaves one tracking waaay off course. I saw some funny results in the XR8 after a few weeks, so now they both cut. I also read that the EEC stops learning when the O2's are disabled... which is what I need for petrol to work correctly.

The Fuel Pump was working a treat when the i6 was in there - and I can hear it start up now. But the whole problem of going massively lean (18's +) on even mild throttle input has me bugged. It will learn around it, but cant keep up with large throttle deviations - and starts pinging its head off. So for now Im LPG only. Im actually going to go get a fuel pressure gauge I think to sort it out.

Still sounds like pump. I've never had much success with fuel pressure to catch pump problems with the engine running. Pressure bleed down over time seems to catch marginal pumps better I find and flow rate(while stressed) better still.
I'd go for a drive and get it hot then pull the return off and check the flow.

LOL. Thats just it - I dont trust it hot. It pings as soon as I touch the throttle.

I'll grab the EF Pump and throw it in on the weekend

NIgel