#1 - The '94 GT did not have GT40 heads (The 5.0 HO is the standard engine in the GT). The heads on the GT HO are exactly the same as the heads that came stock on a '91. The GT40 heads were only oem on the Cobra 5.0 and early Explorer 5.0.
#2 - comparing stock to stock, the GT40 heads are worth maybe 20 HP over normal 5.0 heads (E7TE castings), 25 at most. Combined with the E303 cam you'll probably be in the 275-300HP range at the flywheel if you get the carb dialed in properly on a Stealth or RPM intake.
#3 - Even if the '94 heads you're looking at are from a Cobra, they are 14 years old and probably need some freshening. Look for burnt or bent valves, recessed seats, and/or nicks in the sealing area of both the seats and valves. You might be able to get away with just a 3 angle valve job and bolt them on. Depending on how much you pay for the heads, adding any more work to them will probably bring the cost close to aftermarket head prices which are much better than any variety of Ford iron heads (except for the N351 NASCAR heads sold only thru FRPP).
"It's not having what you want, it's wanting what you've got!" - Sheryl Crow