That's pretty much correct. My understanding (as basic as yours) is that the 998 is a 904 with an extra rib on the passenger side of the case, for added strength. I guess that's a 727 case, as it were. I might have a 727 (heavy duty version; the 904 was light/medium duty and the 998 was rated at medium duty if memory serves.) that was in my race car behind a 360 that had no oil pressure.
I might snag that block (or I've got a couple others) from my house next time I'm in CT and tear that down for a rebuild, and bore it out. A hone, punch and oversized pistons with racing rings should bring the oil pressure back up and run real strong. Unlike my father's current race motor, I plan to do the oil galley mod, because AMC motors are known to starve the rear bearings at high RPM, so running a braided line from the front of the valley to the back and tap holes in the block for the 8AN fittings is the most popular method of getting oil back there. My father simply opted not to do this. Probably because we blew the 304 up and had 1 business week (after a 10 hour drive home) to get a new motor together for the next race the following weekend. We didn't have time to machine the heads for a bigger cam or tap the block for an oil line.
So I'll see what happens. I know 400-500HP out of a 360 is very, very easy. Without even pushing it too far because the crank is still cast. 390 and 401 cranks are forged, but rare. So keeping it under about 6750 RPM you can still make an easy, streetable 400 HP. Even Car Craft magazine did it. I'll dig it out for the parts list. I'll have to make sure the motor has the good heads on it. There were ones with bigger valves and dog leg ports that flowed the best. AMC had the best flowing heads on the market at the time. In these motors, all the power is in the heads. Open them up and let them suckers breathe better and it'll make stupid power.
I can spit you out right here right now a fairly complete parts list that'll net 400+ for probably less than $2k. (Don't know if that includes machine work or not.)
I seem to have misplaced my Summit Racing catalog in my truck somewhere. Most of the parts can be had direct from them for great prices. I've built a race car out of their catalog already (it runs 12's on an unemployment budget,) and have always had a good experience with them every time.
