Probably a 198 rod motor. Purposely built for sure. Anything can be done. I could probably build a 370 NA 273 and spin it 7500 as well. But why? Just to say I did. You can do that easily with a 360 and the right bolt on parts or a fairly mild 4" stroke 318. I guess my point is, a slant 6 is not the most strong block around especially if you're packing air inside.