want to turn engine in a 90 Chevy Silverado "green" - what is the cheapest way to do

A full size pickup was built for hard work, and in the early 90's, gas mileage and emissions were not as much of a concern. Hybrid technology is integrated into a vehicle, so unless you happen to be an automotive engineer, converting it to a hybrid is just not really possible or practical.

The truck would have to be a diesel to run it on cooking oil. I would suggest trading it for a diesel pickup and buy a vegetable oil conversion kit. If you have used cooking oil, this conversion kit is called a "greasel".

And what is your definition of a "green"car? Blue mixed with Yellow? Trade it on a diesel powered one and burn the cooking oil unless of course money is no object and you are only trying to do it cheaply so that you can send more funds to idgent tribes in need of everything. Of course, if you trade it in somebody else will get it so that's not really very "green". I guess you would have to install a diesel to really make a difference. Hybrids generally won't work for most people as they have no power and are expensive to operate and an environmental disaster to dispose of at the end of their short life span.