Roll cages are for protection, any chassis stiffening is just a side benefit.
No, you don't need a roll cage with an sr engine.
89-90 240's and 91-93 240's came with different engines.
SOHC for the first, DOHC for the 2nd...about a 10hp/10tq difference if i remember right. Other than a timing chain guide problem, I don't know of any issues. Tho if your swapping the motor, it doesn't much matter anyway.