On a 1991 Ford Lariat Truck with 5.0 liter...what is the firing order & the
distributor rotation?

Take the cap off for the dist rotation, and the firing order is 1-5-4-2-6-3-7-8

on the distributor rotation, It's counter clockwise. I found a site that has a chart with the firing order and dist. rotation for several engine types and makes I found it at www.summitracing.com/tech/charts_guides/firing_order/tz_02charts_flash.html
edit: seems like the link is bad. sorry for that. but I got it from the summit racing website

dist rotion is counter clockwise, chevyman gave you the correct firing order, on the cap the number 3 plug is the closest one to the front of the engine

make sure to run 7 and 8 far away from each other so that you dont get pre-ignition on #8