As Jay noted, and i have done myself, a little time detailing 390 GT heads, Truck heads (D2T) or C6 heads can easily get you into the mid 350 HP range and with a LOT of torque as Jay noted. Also, some details you must remember to follow though on: zero deck the pistons, carefully work around the cylinder head bowls and chambers, re-curve the stock distributor for more initial advance (12 to 18*) and a total of 34* to 36* range, get stock adjustable rockers if you can, get your cam, crank and rod clearances right, use ARP rod bolts and re-size the big ends, consider lighter forged pistons, run an extra quart of oil in the pan to ensure it does not starve at high RPM on on acceleration, degree your camshaft so you know if your straight up or advanced and are NOT just guessing based on spec sheets... that only works about 30% of the time.
If your really want to go the next step, install a 4 speed TL or a TKO 600... that will make the power more fun IMHO.