I installed carpet for many years and for a time, we carried the right weight and pattern Wilton wool carpet used in KF cars like the hardtops. We also had the commercial level loop in the right grain and color for the rear floor and foot rests on the big Kaisers. I'm no longer in that business...and my knees thank me...but I do know the level loop is still available, even at places like Home Depot. What you're doing looks great and will certainly dress up the interior of the car.
And, Kory is correct, don't try to drill thru the carpet, you'll pull at least one row out of the backing and when that happens, the only fix is to replace the whole piece. A small slit with a sharp knife will be enough to get the seats installed.
Automobile carpet is formed in a press using heat and pressure. Trying to form fit carpet that was meant to lie flat won't end up looking good.