i dont think weight is a issue more engine/drive torque trying to twisting things up
say we took a std book chassis with sierra diff etc then put a big cc torquey v8
the actual engine weight is not a great deal more to deal with but the stress twisting the diff area and chassis will be in the lower gears (gearbox is multiplying torque in all gears below 4th)
pinto engine 98-160NM x 3.58 (t9 1st) = 350-572.8NM
302 v8 5.6 engine 500NM x 3.35 (t5 1st) = 1675NM
other kits have had trouble with cracks and breaks in the rear diff areas so i think some extra bracing will be needed to prevent flex. as the movement will work harden things and crack.
it would not take alot of work to make and fit gusset plates and add extra diagnals in the seatback and diff box areas etc