Mat- you can fix most of the 'difficult removal of the firewall upholstery' by first removing the whole panel. From the back, there are 2 rows of tiny nuts along the vertical sides of the bubble around the water pump. What this means is, the vinyl upholstering around the bubble was done separately from the main panel. Once all the little nuts are removed and the strip of steel the studs are welded to pushed free, the bubble's upholstery can be carefully pulled off from the front, leaving the sides of the main panel still upholstered and exposing the fiberglas of the bubble.
The fiberglas bubble can be sawed free of the main fiberglas panel with a fine wood saw, then the bubble's upholstery replaced on it with upholsterer's glue. The edges of the main panel upholstery can be tucked in around the cut edges and also glued; touch up with a black Magic Marker as req'd. The cut should follow closely to the edges of the factory holes already drilled for the bubble upholstery nuts, and across the top. Now the main panel will bolt into the car and the bubble can be returned to its place using a few pieces of velcro on the steel door. It will look stock but can be pulled off separately from the main panel without tools, to quickly access the steel firewall door underneath. Some add an unobtrusive drawer handle or knobs. The upholstered bubble will fit underneath the usual shoulder bar and can still be removed. There are tech articles on this procedure going back over 30 years.
The shoulder bar as sold by Hall Panteras in CA is a piece of 5/8" thick x 1-1/2" wide mild steel bar stock nearly 4 feet long, contoured at each end to follow the curvature of the window upholstery roll. Hall powder-coats the bar gloss black for appearance. On some cars, one or both bar ends must be rebent (cold) a little so the predrilled holes in the bar will match up to the stock bolts that hold the main upholstery panel in place. Works well with Hall's shoulder straps but is VERY heavy!