I think it's also important to ask, compared to what?
If you're a young guy (or an older one with a bad memory), you may be expecting more than there really is.
A standard 400 CID GTO with an automatic and 3.08 or 3.23 gearing isn't going to be all that fast to someone used to the magic of today's fuel injected, computerized hot cars. Remember, back then, a 14.8 or so in the quarter mile was quite respectable. Today there are stock Honda Accords that can do it that fast.
Fast is a relative thing. Compared with all the other cars in 1969, that wasn't bad. Compared with a new GTO, it's not in the same league.
I'd suggest a dyno to find out what you're really pulling. A stock, 350 hp 400 probably put out around 300-310 net, then by the time it got back to the rear wheels with an automatic, we're probably looking at around 250-260 hp.
Find a club with a free dyno night and get a few pulls. That'll tell you if you've really got a problem or not.