been changing oil every 3000 miles (castrol gtx - even the previous owner did that),
You have hit upon the key item; doing routine maintenance.
A few years ago a car that had been used as a taxi with a 3800 in it was wrecked in L.A. with over 300,000 miles on it. Factory 36 (plant where the 3800 was made) arranged to get the motor back to Flint for teardown. This motor was of interest because it had complete service records. Upon teardown the motor was still mechanically in spec. The crank was gaged on the same jig as the production cranks and no measurable wear was detected. The key was routine maintenance. This motor was out of a cab so it certainly wasn't babied.
The same plant (Factory 36) had a re-union last year with an informal contest that was looking for the 3800 with the most miles on it with no re-build or repairs. The winner had more than 450,000 miles on his motor. With that in mind, as long as you keep the maintenance up, a motor swap is the least of your worries.