If it's one thing Tommy could do it was hit. Spinks could move, but I don't know if he could have beaten a determined Morrison. Tommy had some skill and some speed, too. Spinks was small and didn't seem to take hard punched well at heavyweight. I'd go with Morrison by ko by the sixth round
As long as Spinks moved in and out constantly,he could take it on points. Morrison would obviously have a puncher's chance,though.