When I place my guide, it's not centered on the card

Inkscape and Illustrator show their rulers in different spots (one starts the measurements at the bottom, the other at the top), so my suggestion on where to put your guideline may not be correct based on where your ruler's measurements begin. If your ruler starts measuring from the bottom of your canvas, move your guide to 5". If your ruler starts measuring from the top of your canvas, move your guide to 3.5". If neither of these look right (which can happen if you put your big rectangle in a different spot), check where the top and bottom of your rectangle are on the ruler and find the center on your own.