Hmmm
1981 300 SD Turbo 390,000 miles.
I hope you did replace the solenoid + starter.
The cable from battery to starter may be damaged, but I would check the ground cable from body to transmission bell housing for corrosion damage = very bad connection.
The simplest test for a bad engine ground is to connect battery jumper cables to the negative battery post, and engine block, then try starting, if there is a massive improvement = BAD engine ground = repair or replace engine ground cable, and clean battery to body ground connection point.
Have a great day.
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