I once put a Isuzu turbo diesel in a small tray that previously had a gasser, easy job.

The 320 is a good motor, & would be fine in the 300D body. Pity about the wiring damage in the 300D. If you use the 300D body you dont have to worry about a salvage title, that could add to the value.
The 603 motor is likely to be an internal mess, probably best for parts only.
If it was me & I had the time, the 320 in the 300D body is the way to go, its a newer car as well.

Good luck with what ever you decide!!

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

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