I was in the same delimia about 5 years ago...I put a TPI motor in my '94 Silverado, and couldn't find anyone to tune it....so I started here on learning to do it myself.
http://www.thirdgen.org/promintro
Once I figured out the TPI, then I went on to the GN....now I do the flash stuff with the LT1 and LT4 that is now in the truck.
Your '90 truck will be a EPROM chip, that requires a burner and an eraser. Depending on the chip type, you may be albe to get electronically erasable chips for it from Digikey then you don't need a UV light to erase it.
I use the Pocket Programmer II and Tunercat software for editing the bin files. (links to all of these places and where to buy them are in the link above)
Its not that hard if you have time and patience to read alot on the internet, all the info can be found, and all of the basic principals apply to your truck that are the same for the 3rd Gen F-bodies and turbo Regals since they are all "burned" type of chips and not flash memory like the new ones...(The LT1 cars you only need Tunercat...you can edit the bin file without needing a Pocket Programmer or UV light.)
For Tunercat, go to
http://www.tunercat.com/ and be sure they have a definition file for your truck, go to "Supported ECMs" on their page..if its not listed there, send them an email...hate for you to buy all the stuff to find out no one has a definition file for you...otherwise you have to use a hex editor and know what bytes to change...this would be a royal pain especially if you are not familar with computer hex representation.
Also, seems that the 3rd Gen guys know far more about burning chips and the parms than turbo Buick folks and especially any truck forums.
http://www.thirdgen.org/techboard/diy-prom/
Any questions, post back on here, I'll try to answer them since I've benn here and done that before.
