On my '73, I tried the poly upper bushings and ended up going back to rubber.

On a factory big block '73 B-body, the passenger side inner fender has a HUGE cut out unlike small block cars which have the smaller opening like the drivers side. Because of this huge opening, there really is nothing between the hot exhaust manifold and the control arm.

This extreme heat cooked the poly bushing. It kept squeaking on me no matter how much I lubed it, then I noticed the bushing was actually starting to break down.

Your '78 has a different inner fender structure than my '73, but if I remember correctly, it leaves the upper arm exposed. If you have a big block car and/or are running headers, I'd keep on eye on the upper bushings.