Your automatic is not going to do you any favors, they don't respond to power very well. While you have the motor out, a 5 speed swap will be the most cost effective solution to increase the longevity of your drivetrain. Keep the automatic, and be prepared to replace it very soon.

Are you buying a short block, or building one yourself?

Either way, unless you plan on sleeving I would stick with a B18b. Nows the best time to reinforce your rotating assembly as well. If its in your budget, grab a set of forged pistons & rods and you can easily achieve 350 whp. If not, at least look into having the OEM b18b rods machined to use ARP rod bolts.