There is a good article with a fine plan and detail sketches on the 8inch Mk 5 Howitzer in Military Modelling June 1974.
Regarding the modelling of Artillery pieces: I tend to do it a little bit in the same way that you build WW1 Airplanes: in sub-assemblies, painted separately, and adding the most fragile details in the final process. It often requires more patience than your standard AFV, but it's worth it!
Depending on which mark of 8” howitzer you want, I have photocopies of the scale drawings of the Mk 5 that Peter mentions. However, they’re fairly poor quality, so if Peter has the actual magazine, it would be better if he scans them in (and if you do, Peter, could you email me the scans as well, please? ;o) ).
I also recommend you look at Military-Info, an American website where they sell photocopies of a staggering range of military manuals, articles, reports, factsheets, etc.
Ordnance Dept.Doc.No.1098 Service Handbook of the 9.2-inch Howitzer Materiel Model of 1917 (Vickers Mark I) and 9.2-inch Howitzer Materiel Model of 1918 (Vickers Mark II)(July 1922); 200 pages, 42+ plates of illustrations Price $20.00 {Item No.2983}
I haven’t actually ordered anything from them yet, as I live in the UK and they only accept cheques and money orders in dollars, which are staggeringly expensive to arrange thanks to the venality of British banks. However, despite what they say on their homepage about not sending anything abroad because of increased security since 911, I recently engaged in correspondence with them to the effect that that no longer holds true for materials on old artillery pieces (if you want CBW materials, that would be a different ballgame one suspects!).
PS: I love the way this forum interface changes my fonts into something entirely unexpected, no matter what I set them to...