I don't know, I did this exact replacement in my 1991 GT and didn't have to remove the dash. Oh sure, it would have made it a lot less cramped, but it wasn't necessary and the factory manual didn't call for doing it either, so I didn't. The 1987 and the 1991 are pretty close internally (I should know, I have one of each) so you might get away with not having to yank the dash. I'd say try it without yanking it, and if it doesn't work, yank the dash.
I did have to remove the plastic box to which the blower motor is attached, and I did have to drop down the glove box door to get at all the wiring and bolts. But as I recall there were two bolts under the dash between the blower box and the rest of the venting that could be removed by lying on the floor, and then there were a few more that had to be removed through the glove box. I'm sure there were a couple others as well. It was easier to take the thing out than it was to put it back in, it was a definite contortionist act in the end. But, I definitely was able to do it, and it took a total of about three or four hours as I recall.
Two things to watch out for, though. First, there's a rubber hose that goes from the pressurized side of the blower back to the windings that is used for (I assume) cooling. This hose is not the same for all years, as I found out. The replacement motor I bought did not work with the hose from the 1991, it required the version from the 1987. So be sure to compare the placement of the holes on the new motor versus the old motor. Second, there was one of those push retainer thingiemabobbers that held the plastic blower fan on the shaft of the motor. This thing got destroyed while trying to remove it, and I had to buy a few more at the local auto repair store. I believe they were called "push nuts" when I finally found a pack of them.
Good luck with the project, and may your boys thaw before the spring!