Heightening rough opening for pre-hung entry door

The woes of inexperience. I bought a 32x80 pre-hung single entry door to replace my sidedoor. I've done some minor work on my house before (roofing, light carpentry), so I've decided to try and install it myself. Problem is the 82" unit doesn't fit the 81.5" vertical rough opening where I want to put it. The mfgr, Thermatru, recommends an 82,5" vertical rough opening.
A friend at work had a similar problem and shaved off some of the header. Both a neighbor and the folks at the lumber yard where I got the door say the best solution, though more work, is to cut and remove the drywall and raise the header, supporting it with longer vertical studs.
Has anyone had a similar experience? What problems might I run into, both now and down the road, if I just shaved half an inch to a full inch off the header? I'm thinking that the location of the door is not a heavy load bearing area, so I'd like to just shave the header if it's not problematic, but I would appreciate any suggestions.
THe door is parallel to the joists and rafters. The house is a one-story ranch house. THe headers appear to be 2x6 and maybe even 2x8, on edge. The interior is drywall.
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If your sure the door is parallel to the joists which would mean it isntcarrying any load you can take an inch off no problem. If youve just got attic space above you coud remove an inch even if it was perpendicular to the joists also. You could return the door and get a 78 inch door too to solve your troubles.
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You might also look at the prehung jamb. Often, the side jambs have "ears" to hold the jamb head in a dado. It is possible to remove these ears - may require some glue and nail work, and/or place a set of shims to hold the head in the dado.
I would not tear out the sheetrock, etc to raise the header. You might create more tragedies than you want.
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I handled this situation just last week by whacking away part of the header with a circular saw and a sawzall. I had to hang onto the pwertools because they're not really designed for that, but it worked. I had to remove some nails first so that the saw wouldn't hit them. 15 minutes if you're crafty. -B

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