Scott-
It would depend on what motor you end up using, how much the car weighs, and whether you keep the suspension and wheels/tires stock. The points for modifications all add up and progress you up in class. You would not end up in KM, though, as the SC progresses from I/SS to IS to IP to HI to AM and finally AR, according to the current rules, which may change for next year somewhat. Classifications are determined by what the car is when it starts, not what kind of replica you turn it into.
Check the rules at
http://www.pcasdr.org/tech/rules.html carefully before you finish the build and end up someplace you don't want to be. You will find that using a stock engine transplant is far more advantageous under the current rules than using a modified one. Most of the RS replicas end up in AM if they are extensively modified. If you can fit it into IP or HI, you may be better off.
Good luck,
TT