ive thought about this project myself... maybe cut the B pillar out from each side and replace them in reversed on the opposite sides using at least 5 mm plate under them to brace. (effectively having your rear door hinges facing forwards) Assuming u want a 2 door??? lengthen the doors by offset cutting two doors and cut and shut them. you'll probably need some strengthening up the B pillars now to hold the additional weight anyway! So youve put in a 454 and tubbed it and need help with simple stuff like this?
best of luck