Our cars came with what is known as the Delco 5MT starter. The 5MT starter came in various case/armature lengths and is identifiable by the straight field case. It was a decent starter with the longest case but a better choice is the larger, stepped field case 10MT starter used extensively on gm cars and trucks for a long period of time.
This starter fits in place with a slight trim of the inspection cover starter opening. The 10MT came in a low and high torque version. The high torque model has a larger diameter armature core and thinner field coils than the low torque model. The field coils have two equal diameter copper tabs coming out of the field case where a copper spacer connects it to the solenoid output. If you see a 10MT starter that has two different thickness tabs, it means it has a set of field coils that use two shunt coils to limit the motors rpm and two torque coils, these are not what you want. Two same size tabs means it has 4 torque field coils and is much stronger. A back brace from the end plate thru bolt to the engine block and a solenoid heat shield is a good plan with this starter.
If you want to go the permanent magnet gear reduction route, there are a couple options. A long time ago, a company called Hamburger made a chevy drive end housing for a hitachi style starter and these are ok. Lots of torque but it uses a plastic shift lever and plastic sun gear.
A better option is the Nippon-Denso style, gear reduction starter. These things are bulletproof with metal gears and have ball bearings. The only thing that wears out are the solenoid contacts and plunger disk which are replaceable.