Panel bond and some rivets through the cappings.
On our 90 we just had the cappings off to get some quality primer between the aluminum and steel. Used epoxy on the cappings and self etch on the bare aluminum @ top of panels. Also used dumdum between the two along the top. Took it all apart because we were painting it and changing the rear corner cappings to the NAS style so we can run the 95-97 style lights horizontally above the crossmember. Hard to go wrong there as rover did next to nothing to isolate the two metals there, so anything is an improvement over original.