Hello,
My 70 Challenger has P235/60-15's (T/A's) mounted all around. I haven't had a spare since I've owned the car, but recently came across a steel 15x7 rim that I plan to use as the spare (can't afford the oem "space saver" option right now).

My question is -- given the small size of the E-Body trunk, does it make sense to get the same size tire mounted on that rim (to match the other 4) or go with something smaller? Can you even run a smaller tire like this, if I needed to swap it in due to a blowout, etc?

Thanks a lot!

It really only matters if you have a sure grip and a rear blows out that they are the same "size".
They don't actually have to be the same size, just same diameter.
So you could find yourself a 14 inch rim as long as you got like a 70 series tire, or a skinner 15 inch with 65 series. or something like that.
Or if a rear blows, swap a front to the rear and put the spare on the front.
You could also see about a donut tire from a m-body.

In theory I heard it should be but look at the doughnut tires they provide now a days, I dont believe they are the same size as the main tires. Remember spares only made to get you to a point where you could get your main tire repaired nothing more. Over the years I have changed tire sizes on my cars and retained the same size spare that were in the cars originally, even had to use them on occasion and in some cases they werent even close to being the same size as the main tires on my car and never encountered problem, then again I never drove at high speeds and got my tires repaired ASAP.

get a small spare and as said above,switch the front back if you pop the rear.
i'm using a space saver and i keep a small electric compressor and a tire plug kit in the trunk.

Space saver spares were common even back in the 70's. Heck the AAR and TA's even cam OEM with smaller fronts than the rears. Just have a spare to get you home or far enough to get the flat fixed.