My 1988 or 1989 Ascent uses a 68mm English-threaded BB, and like [MENTION=174646]The Golden Boy[/MENTION] mentions, its length will depend on the crank you want to use. Since mine is a fixed-gear, it's wearing a 104mm BB. YMMV. The bike would have originally had a triple on it, and an RD that would handle at least a 28T cog.

There would be very little point in converting to 700C, even if you "could", since there are really good options in ISO 559 (26") tires.