I found this thread but need further clarity on the exact process having never pulled the engine or tried to align drive shafts.
https://www.socaljetboats.com/engine-mechanical-electrical/converting-bbc-from-3pt-to-4pt-mounts/

I need to replace my driveshaft and figured I might as well convert to 4 point mount with the engine out for ease of use disconnecting drive shaft when running out of water and more rigidity with more power upgrades down the road. I believe I have existing center mounts so I should be able to mount everything without having to worry about drive-line angle changes.

steps:
- mount front mounts to block and drill holes into stringer with engine still installed with 3 point mount
- pull engine
- remove 3 pt bell housing
- install 4 point rear mount on engine block
- put engine back in and attach via 4 pt front mount and existing old center mounts
- mark rear 4 point stringer holes and drill stringer
- pull engine again, remove old center mounts and plug old center stringer holes (best method to fill holes)?
- install engine in new 4 point mounts