The amount of sugar you need for priming is very small, and so the amount of molassas you use won't be enough to really have any flavor affect on the beer. If you want it a little sweeter, add lactose. If you want it a little thicker/heavier, add maltodextrin. If you want both, add both. Boil the malto/lactose in some water for 15 minutes, cool it down, add it to the fermenter. Beer yeast, for the most part, can't ferment those complex carbohydrates, and so it will add residual sugar. You could also add your spices to this mini-boil. But use dextrose for priming, in the exact amount you need (3/4 cup per 5 gallons, maybe a little less for a spiced ale) and don't mess around with molasses. Best case scenario, you end up with a carbed beer that tastes the same. Worst case scenario, you end up with a beer that tastes the same but isn't properly carbonated because the molasses amounts were messed up. Priming sugar is for carbonation, and carbonation only. No need to mess around. I used to get cute with it and try to use DME, but all that ended in was a bunch of uncarbonated batches. So, I implore you, do not use anything but dextrose to prime. It's not worth it!