Just ordered a 5 gallon Black Dog Ale kit from Midwest brewing supplies and I have some Saflager W-34/70 in my fridge. Would this be a good yeast to use or would I be better off with the Wyeast activator pack?
If you plan to brew this as an Ale rather than a Lager then you will need to find a different yeast. The saflager W-34/70 strain is a lager yeast, and ferments best at 12C/54F. This will take longer to ferment and will produce a different beer, certainly less sweet compared to the description on the recipe.
I would get a packet of safale US-05 and ferment with that at 66F/19C. This will give you a clean tasting, balanced ale. If you want more malt forward, you can try US-04, or White labs WLP002. The ferment will also be easier since it will be quicker, and you don't need a temperature controlled fermentation chamber.