The problem with that is finding the 3 hole block brackets is sometimes less than easy, There may be some 3 hole brackets being made aftermarket, seems to me Butler offered some for an F body.
Link to Butler adapters for engine mounting adapters:
https://butlerperformance.com/c-1393...-adaptors.html
There are also 3 different sets of frame brackets, for full size B body, a set for A (73 and later) and F body, then I believe the G body (301) needed a set because they're a much narrower frame from rail to rail.
Also the 65 A body crossmember isn't as massive as the later model frames, and being that Pontiac moved the mount system forward about 4-6 inches, the mounts may have nothing under them. If you're going to attempt doing this on an 65 A body, you'd be in uncharted gray area.
Using the adapter plates at least matches the positioning front to rear to the chassis the engine is going into.
After giving it some thought, the 3 hole mount was used on all 455 A body cars from 70-72. Another way to do this with factory parts would to use 70-72 455 A body mounts, and then come up with the frame mounts for the same car. No they're not clamshell mounts, but that setup was at least employed on the later chassis. There is no guarantee that it would fit a 65 chassis as the engine position was moved rearward in 68, but it would be far more likely than using a set of F body mounts would.
Again finding used parts to make the swap is going to be a problem because there weren't a ton of 455 equipped A bodies made from 70-72. Finding used parts is going to be the stumbling block. Then you may still have the mounts hanging in mid air on the 65 chassis because of the engine setback from 67 to 68 on A bodies.
Either option is going to be try and see if it fits, but you'll have to come up with the stock factory parts before you even know if it can be made to work.