Why would it be so hard to figure out ? Buy a set of blades used in a 72 inch machine, draw out the size of the deck your building, center the middle blade and mark the positions for the adjacent two. Mine miss by about a quarter inch. If you stagger the blades ( one in front, two in the rear ) you may be able to get this down to zero cutting width and still have blade clearance. Look over some decks as they have circles of metal around the blades to create lift and finer clippings.
I have an older deere mid mount deck I bought for fifteen dollars from the junk yard. It is rusted and cracked in areas but the spindles, bearings, blades are good. Some flat sheet metal to form the box and I will have an additional finish mower. ( project number 12 in line ).
This is the best route to go as you have the major parts included. This is where the mig welder shines. Good luck.