I have NO idea how this ended up in the FAQ section without going to the moderation queue... I've moved it to the right section.
The 5M will *fit* in the MKIII just fine, although, I can't think of any good reason to swap it over. You'll have to make some changes (it's not a direct swap) to accommodate the 5M or you could hybridize it as you suggest... for information on that look in the MKII Supra section and at the website
www.supracelica.com.