The change you are talking about must be the change from 1936 top pump engine to the 1937. I'm reasonably sure that by 1939 they were using the 1937 engine (even the few USA style 1939 civilian cars made in England got British built 1937 engines, I believe), and there is a slight difference in spacing between the 2 pads that mount the engine between these two styles. Ford later supplied a very simple spacer that simply moved the holes over and up a bit to mount later replacement engines in '36 and back cars. There are plenty of '36 people here who can tell me if I'm right about the spacing up there, as I have had little contact with 1936 Fords...I hang out in 1932 and 1948 mostly.
I believe the parts variations between 1936 and 1937 English and American engines are mostly slight, and generally USA specs and procedures would apply...the truck heads gave slightly lower compression, there were extra water connections here and there on some military '37's, and someone on here informed me that most British 1937 engines have a longer crank snout like '39 Fords, unlike American versions. This last change was presumably caused by the brief period that England made 1939 cars before war broke out...the '39's had fan on crank and required a longer snout.
Also...I think at least most of the 79 types got locked rod bearings. These are apparently hard to get, but dimensions allow swap of the USA hardware.
