I think I can speak with some knowledge on high mileage 617's.
it is hardly worth the effort to rebuild. Every moving part will be worn. Best replace the motor. You can pull the old motor down for fun if you wish.
What is the trans like? At that mileage it is probably beyond being rebuilt.
Diff is probably very loose.

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1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving