OR you can go to Cummins themselves and see if they can order the manuals you want. I did that although a few years ago and had to wait a few weeks as at that time the manuals had to be copied and printed. They were were no longer stocked. You will need the actual model number and maybe the serial number.
What is the actual model number? 300HP tells us little as there are a few 300 HP engines by Cummins. Mine is the V555M, no turbo, and I have had it since March 1985. I waited a few years to get the manuals that I wanted but i got them.
The other possibility is EBAY. You may have to wait and/or put an add in yourself.
And the membership here will be worth the $50. There are people besides me with some knowledge of these older engines.