I also have the "full size" (MTP-93) Interstate battery in my '87 and it is a very tight fit as well as being very likely overkill for the application. It has lasted quite a few years. (It is not driven all that often and I do have it on a trickle charger when I'm not driving it.)
If/when I need a new battery for that car, I would definitely go down a size, probably to the MTP-91 or whatever the equivalent is these days.
Fyi, this has been discussed previously:
https://forums.pelicanparts.com/porsche-911-technical-forum/650911-1987-911-battery-question.html#post6495086