First off, the block number correlates to nothing that I know of. You should be able to tell whether it's an S13, S14, or S15 by the shape of the valve cover. Duh. We'll just assume that it's an S13 at this point, because they're more common. If you have the original O2 sensor, skinny=redtop and fat=blacktop. If it is a blacktop, then you can tell if you have a late-model 97-98 motor by checking out the timing chain tensioner. If the bump on the tensioner is the size of a dime, then it early. A bump the size of 3-4 quarters means that it's a late model. Back to redtops. They will, of course, all have the small bump on the tensioner. The redtops also have a subset, where the later ones got the 54C pistons. If you're good, and the motor is in decent shape, should be able to read the piston tops through the sparkplug hole. I can't remember the other, early stamping off the top of my head. Blacktops will also have the 54C piston. There's also talk about figuring early vs late redtop by whether or not the front of the head has ridges or not (smooth front=early, ridges=late), but I can't confirm that. Of course you can also tell by the large ECU number if you have the correct one. There are other tricks, too, so lets hear what your motor has so far.