Frank, unfortunately there are no simple answers for you. Unless you have a lot of fabrication and engineering experience, changing the drivetrain is a near impossibility. The Kent Cortezzes use an Olds Toronado drivetrain. While some have converted the Clark models to the Olds drivetrain, it is a complicated process, which requires refitting the entire Kent cradle into to the Clark. Jerry Mcmurray, who frequents the yahoo group, fabricated a custom bell housing to attach a Chevy 350 to the Clark transaxle. But again, this requires a certain level of fabrication and engineering experience. Also adding that extra weight to the front suspension and drivetrain has it’s own issues.
Your best bet would be to boost the power of your slant six. Take a look at slantsix.org. Those guys are the most knowledgeable about anything slant six related. Otherwise, if you can’t live with the lack of power, I might suggest that a Clark Cortez just isn’t the right coach for you.