I disagree with taking the body off and upside down

It should be repaired on the frame where it is with everything fastened down so it will be properly stressed for the alignment

I also have an early Gibbons 33 and to get the floor around a 4l60 trans I eventually cut out the hole big enough for the trans

I then put a piece aprox 2 inch foam rubber packing material out off something that came into the shop, covered with wax paper and started laminating a new hump over the trans--this game me about 2 in clearance and minium obstruction inside the body--I laid several layers of mat/ woven glass and took my time ( got to savor the fumes) a small heat lamp hanging inside the body helped with the set up of the resin

All door opening gaps/alignment remained the same

Jerry