Check with your state's Extension Service. Each county should have it's own Cooperative Extension Office which provides free publications and information for the asking. They will have valuable vegetable/gardening tables available specifically for your area determined by universities and horticultural research scientists who have collected data from growing those crops in your state. Your tax dollars are already paying for this service so you may as well get some use out of it. See how detailed these examples are: Vegetable Planting and Planning Calendar for Missouri (download the pdf) complete with spring and fall planting dates (underneath the spring planting dates for appropriate crops), how much to plant per person, etc. They can also tell you the average last frost date for your area and ideal planting times for specific crops and varieties in your area. Here is an excerpt:

It's a little late even in your zone but they will still probably do ok. You may get a limited crop but it's definitely worth a try. Dave

Yukon Gold is a mid/early - about 70/80 days or so. Fingerlings are usually late season - something like 90+ days. That being said, even if they don't produce tons of tubers, if you can keep the soil cool, you should get several pounds of tubers for each pound planted. They don't like to produce tubers if the soil's too hot, so if you can keep the soil below 75-80, you should get a decent harvest.

I'm not from your area, but you should be safe to plant potatoes after your last spring frost. Your main concern is to have them mature enough before frost kills them. In 8a you should be fine, even with late seadon varieties. Good luck! Here is a link that might be useful: Sinfonian's garden adventure