Regular hockey puck from Amazon or your local sport shop. Think its a 5/16" cap head bolt. 3/4 in length. (That is a 5/16" allen wrench), so it may be a larger diameter bolt (3/8") Either will work.
Put blue painter tape(or duct tape, any tape) on your drill bit to match depth of bolt. Find center/ drill hole. Use allen wrench to screw bolt
into the hockey puck. Good for 05/06 GT.
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I've been using a floor jack and jack stands for 19 years to work under the GT. Yes, Pete, a 4 post or two post lift would be nice.
The wheel cribs you posted will work for getting the car up to get the floor trays off and doing maintenance, unless you need to do suspension, steering or brakes.. Floor jacks work for almost everything. Lifts are even nicer.