Exterior door replacement trim question

Hi everyone, I'm about to replace an exterior door with a pre-hung Therma Tru door. One thing I'm not quite sure about is the trim on the outside of the new door. The old (current) door has thin ~ 2 inch outside trim that the painted clap board siding buts up against. The new door has much wider outside trim.
What's the best way to deal with the clap-board siding? I was thinking I might be able to nail back the siding, then use a circular saw to cut away the couple of inches of siding needed to fit the new door & trim. Trouble is, I won't know the exact placment of the door until it's in place, leveled & shimmed.
Now that I think about it, I might be able to remove the trim from the new door for the install, the replace after I cut back the siding? I hope the trim comes off. Sometimes this stuff is glued and staped so much that both sides get wreaked trying to remove just the outside trim.
Any advice would be appreciated. I've never replaced a hung door before.
-John
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That is what you want a tight fit between the finished siding and the trim. You are going to set your door plumb and level. So if you cut the siding opening plumb and level (and the correct size) the only way the door will fit in it is plumb and level. The only "adjustments" need will be to shim the door jamb/sill so it fits into the newly cut siding opening.
Would it be unusual for the door opening be framed so that it

Unfortunately no, I have run into this situation many times with Therma tru doors. I have had to cheat and cut a 1/2" off the header. (Shame on me).

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Any advice on the best tool to cut back clapboards? I was going to get one of those little 18v battery circular saws. I'll snap a line, then nail behind my line, then have at it.
I'm not even a weekend warrier at this stuff so any advice would be appreciated.
-John

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Most therma tru doors have 2 1/4" brick mold same as most other doors. Yours must be special ordered with the larger trim? Any way a few solutions: You can just remove the trim, install the door, and cut down the new trim or reuse the old trim. You could also remove the trim, install door, and then cut back siding and reinstall new wider trim. You can leave the new door intact and cut back siding first. Just lay out level lines, the size of the complete door unit, from the RO center and cut. New door will fit into this new opening and be level. This is the best way IMHO. The problem I always have with therma tru is they are 1/2" taller then most other doors.

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