I used the low side pressure switch from a mid 80s K car to control the OD solenoid on my truck. The one Napa carried apparenntly has a higher setting than the brands you get at other stores.
It should fit in your truck just fine.

Here's my write up from another forum with pics and part numbers: http://www.dieseltruckresource.com/dev/another-way-fix-od-t156988.html?t=156988

We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon