Thanks all, really appreciating the advice and suggestions. I have actually built wooden rim/tire mock ups to check for clearance etc etc. At this stage 255/60/15 look just to miss everything and 235/60/15 on fronts which again look to just miss everything. I have some old stock 15" rims, so I might take a punt and buy one tire of each and fit to these rims and pack them out to replicate my estimated offset. That should be the safest direction forward yes?
Psycho - very jealous you can get 295 on the rear! Just too big for my DRB build space