Well this reference should be of some assistance to you as it addresses both the 2, 6 and 17 pounder guns carried on British tanks. Also you'll note with the Churchill MKIII the use of the Sherman 75mm from damaged and destroyed U.S. tanks. Also covers Firefly and some "offshoots". Though not how or where I like getting my references from, it is in line with what I've come across from some British unit, other game sites, Arm Chair General etc. and general info sites/blogs in my search. This one just seemed to tie it all up in a neat package.
http://www.h2g2.com/approved_entry/A30561013
Taking a short break from next door and catching up on my reading before
"THE GUNS AT LAST LIGHT" comes out then I'll take a break from everything...sort of.
Regards,
Pat