The throttle body opening is definitely a little smaller than 70mm, I had to take off maybe a couple mm all around. I did some measurements of the cross sectional area when I installed this stuff.
Stock '91 TB: 65mm diameter (as measured), x-sectional area: 33 cm^2
BBK 70mm TB: 38.4 cm^2
94/95 opening to supercharger: 104mm x 37mm with rounded corners of 7.5mm radius: 38 cm^2
On my S-model, the opening on the supercharger was larger than the plenum opening, so I opened it up by a couple mm all around. Still, I think the internal size of that plenum is such that a 70mm TB is probably all you need. With a bigger Magnum Powers plenum you would see more advantage from 75mm or 83mm TB.