I've got a similar duration roller cam in a magnum headed 318 in my 4000 lb 5th ave. I average 17.5mpg mixed with it.

an '87 318 in a truck will be flat tappet, but should be a roller cam block. I'd look at the smaller voodoo roller, too...contact board member scratchinfortraction for details. he might have a used voodoo roller cam for sale, too.