The internals of a 400 HO are the same as the internals of a standard 400.
The cam, carburetion, and distributor curve vary from year to year and auto to manual.
Bolt on accessories are mostly identical. Check the AMA specs on this site, and the sales brochure powertrain section, to note any differences.
Purchasing the correct numbered carburetor and distributor can be a lengthy and expensive search. Some carbs and distributors were also used on the standard 400 configuration and are more common.
If the numbers matching block is already .060" over or has badly rusted cylinders or is cracked or damaged, a generic rebuilt Pontiac engine may be a reasonable compromise. Pontiac does not have "crate" motors.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
