The frame rails are not the biggest problem, the location of the steering box is. It sits inside the frame rail on the drivers side. The 48 only had an inline 6 or an inline 8 motor offered, so steering box was not an issue then. Its the "V" in a "V8" that causes a problem with the steering box. You can gain enough clearance by shifting the motor & trans towards the passenger side of the car 1 1/2" to 2" on a small block. I personally have not installed an RB into one of those cars, but its been done several times, even 1st & 2nd gen Hemis have been installed in these cars, so it can be done.

I've installed rack and pinion steering in Plymouth versions, that solves the steering box issue. Gene