I know I need the tuner and mounting plate, but I've heard that I may also need a 20" 235 exhuast tube because my 150 has a flat back and the 235 has a bubble back exhaust. Are there any parts I'd need in order to swap both powerheads that wouldn't be already within the two complete engines?

Start:
1990 20" 150 VRO
1985 25" 235 VRO

End:
20" 235 VRO
25" 150 VRO

Thanks.