I was originally planning to leave the original Hardura insulation on the tub bulkhead in place, partly as an authenticity thing and partly because removing the black tarry glue that was used as an adhesive is just so unpleasant. However, the bullet has been bit, and it will be a good thing overall as the entire tub will then have been inspected, repaired and painted.
After about an hour of scraping the glue, it suddenly occurred to me that Eastwood's Pre also claims to be able to remove road tar, and the adhesive has some decidedly tar-like qualities. And so I tried it, and lo-and-behold it did the trick, albeit it took a lot as the adhesive was very thick and the Pre just took off one top layer at a time. However, in the photo below, you can see that the upper half of the bulkead has been cleaned in just a couple of hours work. Mineral spirits were used for the final surface cleaning.
In this view, the Hardura felt insulation lies on the ground, having been carefully scraped off, while the remaining scrapings are to the left. This piece of Hardura insulation will be kept, because while there are is a Hardura replacement kit out there, it doesn't appear to include this piece. Hardura felt itself can be ordered, and perhaps the local upholstery shop will be able to make a nice new duplicate.