Duluth Longtail T-shirts are a nice middle ground between "regular" and "tall".

I'm not quite tall enough for Tall sizes in most cases, but some "regular" sized shirts are too short for me. Duluth t-shirts are just right. So they may be long enough for your purposes too.

(They do offer Medium Tall in some of their other shirts, but not their T-shirts specifically.)

Also note that their standard Longtail T-shirts are cut baggy, so you may need to get one size down from normal, especially if you're on the edge between two sizes. Or, they recently started offering Slim Fit Longtail T-shirts, which may be an even better option for thin guys like you.

Stop by a Duluth store to try on some different sizes and cuts, if you can. (Looks like there are stores in both Pittsburgh and Philadelphia.) Otherwise, their free mail order exchange policy is pretty painless.