To replace full torque box the front floor and toe board need to come out. I guess you could just do the bottom of the box, but you generally run into rotten rockers and frame rails that require repair also and its easier to replace everything at one. Just my opinion
Question, why are you REPLACING a torque box on a 65, I thought 67 was the first year for that and they only had it on one side.
You have to do the entire tb at once, you can't just replace part of it otherwise the structural rigidity of the box will be compromised and it will do nothing for you.
Judging by the way mine went in, with a good cut off disc you should be able to just remove the old box without disturbing the surrounding sheet metal and then pop the new one in:
The PO on my car had installed some additional metal and bracings on the sub frame at some point so we had to grind through them with the cut off wheel and angle grinder but it only took a few hours to get that offending metal out of the way, to cut through the floor in the correct spot, and to have the area prepped
The crap we had to remove:
65-66 Convertibles do have torque boxes. They are actually reasonably easy to replace. The toe boards don't have to come out if they are not bad but I always had removed them and welded the remaining floor to the top of the torque box so I didn't have sandwiched metal, prone to collecting water and rusting.
Fun days ahead for the convertible. You can replace just the torque boxes, but keep in mind that the inner rockers rust from the inside out. When I pulled my inner rockers out, I was amazed at how much rust there was. Sure didn't look that bad in the car.