So answers so far are:
Q1 - I'll give Git 1/2 point for the 466ci 6cyl diesel. Answer is 500ci diesel as fitted to 1975-1979 ELR and EMT Bedfords. Biggest 6cyl petrol engine is a 300ci engine as fitted to EFR Bedfords 1970-1974. This would have to be close to the biggest engine in a GMH complianced vehicle too? How big were the Isuzu truck engines?
Q2 - no answer yet. Warren has the capacity right, but the blue flame was only a 235ci or thereabouts. What had a 250ci 6cyl?
Q3 - HQ to WB doors with no intrusion bars. 5/71 through to approx 12/84.
Q4 - HQ to WB cowl grille. 5/71 through to end of WB Statesman (roughly 1/85, maybe fractionally later).
Q5 - HD to WB van roof turret panel.
Q6 - Speedo cable hole in firewall plus small hole in passenger's floor for cable support.
So anyone care to expand on Q2?
While researching Q1, I actually found another good question - what is the biggest capacity V6 installed in a GMH complianced vehicle? No-one will get it as it is pretty obscure. 1968 ERV Bedford had a 6V53 Detroit Diesel, 1970-1974 ERV had a 6V53N. A 6V53 is a V6 diesel (see here
http://www.army-guide.com/eng/product160.html). Being a 2-stroke i'm not sure if the formula for 6 x 53ci = 318ci actually holds true?
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If we all had the same (good) taste, who would buy all the Fords?