Subframe connectors for a driver/cruiser, necessary to weld to floor?

If you have the means to do it then go for it. I think everyone (including myself) pretty much agrees you don't need them but they will still make a difference. Home-made steel box tubing worked for me (not welded to floor) and made a noticeable improvement in how solid the car felt. Not just in handling but going over bumps and opening and closing the doors, the whole car just feels tighter and more sturdy. It won't have an effect on how good your burnouts are lol but you'll appreciate the feel of the chassis being stiffer.

Thinking out loud about torque boxes, for one they're much smaller and cheaper to make and install compared to subframe connectors. IIRC they were also already designed for convertibles before the Street Hemi came out so 2 reasons there for the factory to do it. I imagine it helps beef up the area around the front spring perch which is seeing all of the forces from the drivetrain trying to push and twist the car, but when things get more serious the unibody between the front and rear subframes can still start to buckle and twist which torque boxes don't really help. Or maybe they do because they were originally designed for convertibles... idk lol. I do know my MP Chassis manual mentions subframe connectors and that pretty much any unibody Mopar will benefit from them.