Before you weld anything check the fit of the doors to ensure the body has not bent in the middle because you cut the door opening out.
You may have to raise the rear slightly to close a gap if it did.
Do It this way.
drill the holes for the rear bumper tubes (to the size of the actual tubes) jack the rear up or use cargo straps to pull it up until the door gap is where it should be.
Mount the rear bumper tube mounts to the frame and the bumper tubes to the mounts to hold the body in position.
I have a customer in Lake Havisue (sp?) that had his cabriolet doors professional done (by persons that shall remain nameless) and had to completely redo the entire process (by himself including welding) because the door blanks were cut out while the body was still movable.