$300 for valve adjustment sounds very high..... even at $85 an hour. Shouldn't take more than an hour if they have never done it before. There are other Bay area members who can probably point you toward a cheaper shop... although it sounds like this one may know it's stuff.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k