How connect steam Cast Iron baseboard around 90 degree corner?

What about expansion/contraction?

This is a replacement situation. 16 feet of cast iron base board now fed by one pipe steam system. Supply is 1 and 1/2 inch steel almost flat run out from main line. This run out also supplies another small rad on the floor above. The existing base board has 3 cracks, 2 at the far end and another at a point just before the inside corner. Per Dan's book TLAOSH I plan to add a 3/4 or 1 inch (which is best?) return line from the far end of the baseboard back to the wet return. I'm wondering why this baseboard cracked in the first place. The two 8 foot pieces are joined together with 3/4 copper male adapters and 90's. How much will this baseboard expand? It is held by the exterior wall one one end and I can't see the pipe on the supply end because it comes horizontally out of a cabinet. Thanks for your help!