I would NOT use that gas. Drain almost all of it out then put in fresh gas. I like the idea of removing the fuel filter and turning the key. The only concern is if the fuel pump continuously pumps with the key on.
You should check if another drive shaft or any other parts are needed. Instead of buying from ebay, check out ddperformance.com. Seem to have a very good rep. They sell T56's. Very informative website. Also compare shipping costs.
Why bore? If you keep the stock bore, you'll have enough left for an extra rebuild. I don't think there is a different price in pistons but I never checked.
I would go with the DSS. I am in the same boat as you, looking for a shortblock. I was originally settling for the d&d but have decided when it's time, I'm getting the DSS 331 pro bullet with the level 20 block. Seems like high quality stuff!