Just curious, if the distance between the meter pan and the breaker panel has to be "the shortest distance possible" , why not replace the entire length of cable ? I know I don't have all the particulars of the job but it would seem to be easier to replace the cable than it would to mount a 6 x 6 box with w/p connectors and butt splice it in the box.
BTW, NSI and Polaris make insulated butt splices. I would look into one of those before I used heat shrink and std. butt splices. A few $$ more but well worth it.