Installing interior carpet

I'm going to start installing my interior in my 66 Dart. Here are some things that will make this a little more difficult. First I have installed a rollbar. I have the front bars welded to the floor in the front floor pan area. Of course they tilt toward the rear so I will need to cut holes into the carpet to slide them down over.
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Are there any secrets on how to know where to cut the holes because as it slides down the bar it will be going forward. The hoop bar isnt welded to the floor yet because I thought I could slide the carpet under the bar, mark the carpet, cut the hole then put the carpet in and have someone hold carpet up while I weld it into place.
When cutting the carpet have you use a utility knife and do you heat the edges so they dont pull or run as they get older?
The carpet is new and will probably be stiff with crease from being in the box for over a year.
After the carpet is the headliner, that will be more questions!
Looking forward to hearing your thoughts.
Rod