Early bikes used regular old motor oil in the forks which is going to be far "thicker" than 10 weight fork oil.  I like straight 30 weight since it's hot here.  Probably won't solve the bottoming though.

You could try Race Tech for springs.  Cannon Racecraft will make anything you want for springs.  Also see notes from RT325 on spacers etc and sometimes manuals will have specs for spring free length so you can tell if yours collapsed. 

If you're handy, cutting a few coils off the stock springs and adding a solid spacer to make up the difference will increase the spring rate for almost free...  you have to be careful not to cut too much and get into "coil bind" RT325 mentioned. 

Just find some plastic water pipe, same diameter as the springs, and make some simple pre-load spacers. Its possible to get damper rod forks, working far better, as outlined in my earlier post, but it's a fair amount of work, and may well mean getting springs custom-made, for your rider weight.

Racetech does make emulator and springs for the '74 DT360A, I do not see them for the 175. Rather than give an amount of oil per fork, they are using a distance from the top of the tube to the oil using a tool they have developed, which I have seen a MotionPro version of as well.

Anyway, more info is better?

RT - Digital Product Search (racetech.com)

Leading Link bikes have front shocks. About 4% total.

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