Cannot help with the dimensions but this will give you an idea of the style:
https://ccmv.aecsouthall.co.uk/p671095361/h2d19c300#h2d19c300
Which version do you have, there was a 10 ton GS cargo FV1121 as per the link, a long wheel base version and a tractor, GS for 8" howitzer FV1122 that had a full length canvas tilt at almost the same height as the cab roof?
This is my old one, bought from Witham in 1988 with just 1200 miles on the clock and never had a load in the back.
Edit. Apologies, I completely missed 'artillery' in the title'.