My first foray into the mount adapters was the same engine swap, A 69, 428 into a 1973 Esprit, circa 1977. I made my own adapters from steel strap. I sold them for $20 a set back them, as it was the only game in town at the time.
I had a service station/garage in Erie PA, (Yost Pennzoil Service) that specialized in Pontiacs, especially high performance Pontiacs. In my third signature picture, you see the light brick building behind the 69 Grand Prix dirt car, and see the advertising on the front fender.
The word got out that if you had a Pontiac that didn't have the proper 5 bolt dual fit pattern block, that I had adapters that would allow you to make the swap. I kept usually 1 of each set late to early, and early to late, hanging on the wall for people that had run into the mount swap problems. I probably sold a dozen sets during the late 70s, early 80s time period. After the lease was over (1982) the oil company tore down the old service station, and put up a new self serve kiosk station. I still had people seek me out to buy a set of the adapters for years after I left the service station.
The problem with the thin strap iron was that under severe usage, the mild steel would tend to deform. I never had any breakage, but they would deform somewhat, and bend.
