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I don't see how a single 4 or 5 foot blade out front is going to solve your problem, but there are numerous front mounted mowers out there from Toro, Deere and others which are sold by golf courses, etc....Even those will have multiple blades with each one no longer than 24".......
Imagine trying to mow with a deck out in front of you large enough to handle a single 4 or 5 ft blade. Seems like a potentially troublesome task.......The mower would be 5.5 ft deep (front to back)at a minimum.......