If there's nobody nearby to let you sit in one, you could always find a dealer with one and feign interest...
You might well be a bit tall for an SLK. I'm going to see (and probably buy) an R171 SLK55 on Monday; see my thread 'SLK Versus SL' in this forum for the full story. I'm six inches shorter than you, and quite chunkily built; narrow barn door rather than full-size.
Quotes from our transatlantic cousins' website:
"I'm 6' 4" and 290 pounds and I can sit comfortably in my SLK 230 Automatic."
"I was looking for a Porsche Boxster, tried one, managed to fit in it, but as I was leaving the dealers I saw an R171. I had a sit in the SLK and compared to the Boxster there was so much more room, I'm 6 foot, 23.5 stone ex rugby player and ex doorman, I'm not small. So I bought an SLK350."
"I sat in an SLK200 today and AMAZINGLY there is enough room. I have 34" legs and with seat all way back and down I have enough room. I moved seat forward a bit and moved seat back further and still had leg room without touching anything. 1" of headroom and not too bad to get out of."
In your case, it will I think be tight at best. Good luck.