I say King Ralph because he and Harper were probably the only 2 electable Blue Tories nationwide at the time.
Where do you get the idea that Klein was this popular? By the time the 2004 had rolled around, Klein's popularity was declining in Alberta- which is much softer on conservatives than the rest of Canada (generally). His whole campaign against the recognition of same-sex marriage put him at odds with a lot of the voter-base in places like B.C. and Ontario (while Harper would later keep a lid on the issue). In addition, only two years later in '06 he'd barely hang on to a leadership vote where he managed a whooping 55%. He also would've been grilled on the national level about his alcoholism, his style of government in Alberta and probably with his Hell's Angels photo that even dogged his time in Alberta.
Klein was very popular for a time, but I think if you want him to go for PM you'd have to have him go at it much earlier than '04. If you don't want to start hand-waving stuff anyway.
I'd also think instead of Klein that was electable it MacKay that'd be the #2 most electable Blue Tory in the country.