Took plugs out and it will turn over, but can't turn it over with a socket on the crank. Tried and so hard it feels like I will strip the crank bolt before it moves.

Put an inspection camera in the plugs, look fine. One has these strange looking white tear drop shape markings (but not deep, can't see any depth to them at all)

Took valve covers off, they look fine, turned it over a few times and watched them, looked fine to me.

Put it back together and it started, but ran for a couple of seconds and stopped suddenly again.

Sounds like soemthing is eith putting so much stress on the mother it can't keep running. Or something is jamming up causing a seize like effect.

This is the 1990 454 magnum with bravo drive

Pull the drive and see if it turns easily, might have seized up a drive. If not, don't worry you probably needed the drive off anyway to yank the motor.

Whatever it is, I promise it won't be cheap

If the drive is seized, would the prop still spin? The prop spins fine, and when put in gear, it engages. The boat is on lift in boathouse in above the water. Removing the drive will be difficult.

Where on the drive could it be seized up? Would taking off the lower unit upper cap show me anything? (I did crack those gears years ago, and that made a very loud sound, but the motor still ran ok)

Can I loosen the motor and pull it forward off the drive spline, and see if the motor runs ok? Or is that much more work than I think?

Hope your wrong, but I think your right!

Nope,.... The drive comes off 1st, then the motor is lifted straight up, then out,....

B-reak O-ut A-nother T-housand

it is a crank-rod bearing. sometimes they lock up without knocking. cool down and spin again for 30 seconds. a single thousand isn't gonna do the trick unless OP can do his own engine work...

I think you can give up on the pipe dream of fixing the boat in the water. Your options are basically a drive issue or a motor issue. Either one isn't getting fixed in the water. Step one would be getting the boat on the trailer so it can be worked on.

I'd bet the farm on it.