I just finished installing tri-power on my 66 GTO. The clutch began to slip during the test drive, but now it is getting much worse. I believe it is the original clutch from the factory. Does anyone have a recommendation for a replacement? What about the pilot bearing? Obviously I want to get this done right away so I can enjoy the car.
My recommendation would be a new clutch.
There are many brands out there and they typically will offer a horsepower rating so you can kinda match it to your car. Bigger HP requires heavier pressure plates and you don't want one you have to use both legs to push in.
The pressure plate is typically a diaphragm style, but you can opt for a Borg & Beck, or Long with the latter for HD applications, so look for a diaphragm type.
What you want to look out for is that if you buy a kit, they sometimes give you the wrong length throw-out bearing. Many give you a CHEVY which seems to be the incorrect length. Some diaphragm pressure plates use a flat finger design while other use a raised finger design - so a flat finger would need a longer throw-out bearing length than a raised finger design.
You do want to replace the pilot bearing. I prefer a solid bearing over a roller bearing. Reason being is that a roller bearing can fall apart if it wears out and some of those needles or ball bearings could impact your clutch set-up and do damage.
While you have the trans out, I might have a new front bearing installed and front/rear seals.
Brand is one of opinion. You might check the assorted Pontiac parts suppliers as they all offer kits for your application. Summit/Jegs will also have kits. I have used McLeod http://www.mcleodracing.com/index.php/clutch-kits.html
in the past. Clutch discs come in a variety of materials and types. For most street applications, you probably want organic. I tried a Kevlar disc on my last clutch and it did the job, but it did have some "chatter" when letting out the clutch from a dead stop - not the smooth slip you get from an organic disc. Here is an article that may help Choosing the Correct Clutch - Hot Rod Network
So, don't put too much thought into this and get bogged down with all the choices. I think a basic clutch set-up for the street and your HP (rather torque). You can always email any clutch supplier and see what they recommend and ask about pedal pressure. Diaphragm pressure plates have easier pedal pressures.