Considering that it is a 78' block, it very well could still have the low compression pistons. Compression could be down in the range of 7.8:1 or a tad higher. If so, 387 hp is actually a fairly decent number. With the right gearing and torque converter, you should stil easily be able to achieve 13 second quarter mile times, maybe quicker depending on the car. Id use a stall somewhere in the range of 2500-3000 and a gear of now higher (low numerical) of 3.23. Id be very happy with this motor in a street cruiser, with some trips to the track.

There may be a few horses left in the motor with a slightly different cam, like a XE268 or XE274. More importantly, you may be able to get the low end torque number up with a slightly, without affecting horsepower much.

I say run it. You still have a healthy motor and will light the tires up with ease, especially with the right TC and gear.