I now have a few thousand miles on the new MP Magnum 360, and the same exact thing happened with this one as the one I installed in my '85 several years ago. For the first several hundred miles, it almost seemed like what I would call a "dog". You could feel added torque a bit, but it nothing impressive. Then, somewhere around maybe 1500 miles, it really started waking up. Now, holy cow it is fast, and I still have the 2.94 gears. Just cruising at around 40mph and giving it a little gas, you can see and feel the front end lift and it pulls sooooo strong. These really are impressive engines for the money IMO, and I highly recommend them to anyone looking who can find a nos one and doesn't have the tools, facilities, and/or desire to build their own (I just saw another on craigslist a few months back for around 3K, which is what I paid for mine).
I know a engine is supposed to gain power as it breaks in, but I didn't think it was normally so drastic of an increase, but I am not complaining! I love it.