"Your engine may well be a 427
"Your engine may well be a 427 and not the 390. I was aboard a 427 Century not long ago, and they are known for using the 427 (too).
You can get a positive ID on that, by looking at the oil pan, and just above the oil pan if you see three large cross bolts on each side, about one inch above the oil pan gasket, that is a positive 100% ID that it is a 427 motor.
A 390 is still an awesome motor that should remain in the boat. Many people are tempted to use a more common Chevy motor, but it will ruin the ultimate provenance of the boat.
Here is a link to my personal collection of 427 info, much of which can be used for the very similar 390. The motors are using the same crankshaft, and the bore is bigger on the 427, along with the use of cross bolted main bearings. Just about everything that fits on a 390 will fit the 427, and vice versa.
http://www.network54.com/Forum/503931/thread/1149537911
In addition, I also have some Interceptor info, and it can be found here.
http://www.network54.com/Forum/424840/message/1143004270/
Here is a thread showing the Centrury with Interceptor 427 power. It's still up in the air whether or not this is a 300 or a 330 as the owner claimed.
http://www.network54.com/Forum/424840/message/1174882540
Here are some vintage Interceptor scans from my personal collection
http://www.network54.com/Forum/424840/message/1184015166
Hope this helps ya!
Regards, P"