Thanks John, I appreciate the suggestions and warnings.
At this stage the only thing in the cockpit with me are the brake and clutch master cylinders. Nothing forward of the foot box structure. Of course, when I start to squeeze things in rear of the firewall the space may dictate different solutions than those in my head.
Finalised the inner lining last night and bolted the harnesses in. Strapped myself in as tight as I could and made vroom vroom noises. Couldn't actually reach the turnbuckle to release the harnesses - my hand didn't quite turn back on itself far enough with the shoulders so completely isolated and no way of nudging an elbow out the side now the sides are in.
I 'lay' there with the vroom vroom noises gently turning to a whimper. Apart from the obvious bail out fail I was wondering how long it would be before Tracey came out to say it was time to come inside - she doesn't mind me spending time with the mistress in the garage but a bit at a time, not hours on end.
Then I clocked that the shoulder harnesses could be easily slackened off in that position (only needed to slacken one) and that freed my arm movement to allow my hand to do the buckle - a 2 second release process so that should be fine (?).