The reason it is difficult to find a Pontiac GM high torque starter today is because they all have worn out armatures and have been rebuilt to death. You can only resurface the commutator on a armature a few times, then the armature must be replaced with a new one. New starter armatures have been discontinued from GM since the mid-1970s. That is why new aftermarket starters have become available from Chinese manufacturers. My 455 engines were 1963 421 blocks which used a bell housing mounted starter and all of those were high torque starters. The low torque starters were first introduced with the Pontiac 350 and 326 V8 engines but starting in 1973 through 1979, all Pontiac V8 engines received a high torque starter. You had to be careful when you purchased a 1964-1973 Pontiac starter because you may get a low torque starter. The 1964-1979 Pontiac high torque is about 2" longer.
My 455 CI (1963 421 blocks) Pontiac engines used a bell housing mounted starter and they were discontinued from the local parts stores in the mid-1980s. I had to locate a new armature from NOS parts vendors and rebuild my 1963 starter. All the the parts needed to rebuild a GM starter are still available from aftermarket suppliers except for the 1955-1963 starter drives. I managed to obtain several starter drives, commonly called a "bendix", and three NOS Delco-Remy armatures for 1962-1964 Pontiac engines. I also used AC Delco starter solenoids with the brown cap. Those HD solenoids were first introduced in 1970 for the 455 Pontiac engines.
As I wrote before, my starting problems with Pontiac 455 engines, 12:1 compression ratio, 9.8:1 dynamic compression ratio and 220 PSI cranking cylinder pressure was caused by modern batteries that are designed for engines with less than 9:1 compression ratio and less than 350 CI displacement.
I use a starter on my 1991 4.3L V6 Chevy engine with 10.13:1 compression ratio (8.4:1 dynamic compression ratio) that is designed for a 7.4L V8 Chevrolet truck.