Good question, I have a MR 19 long cargo with a full length rack H2 Ladder Roof Rack For Ford Transit Cargo Van 2015-On 149 WB – VANTECH USA INC.
From what I know the limit is only 450 pounds for the Transit roof, I've had about a 1000 on mine. And no I don't think the mounting points are part of the crossmembers but don't quote me on that.