I went with the 13 x 1030, and it seems to fit fine. My 1985 300D with the dual pulleys uses the narrower 10 x 970 belt.

Got a crappy alternator from Heartland Starter (on Flea Bay), and one of the pivot holes (pulley side) is too large, so it sat crooked. Shoulda sent it back, but it was such a PITA to install, so i shimmed it with some flattened folded over TV antenna elements. Hope it holds.