I will stay away from the cross over setup. I really like the looks of the stainless ones but looks and function are two different things.

BBB, will the stainless Sierra housing work fine with an alpha setup?

I am using the one from S/M and I can say it is a nice unit

It should work fine . Make sure you have the 3/4" water tube in the transom assy. If not the older alpha's had 5/8" tubes and also the fittings in the bellhousing and hose were smaller. Make sure pump is perfect in the Alpha. make sure there are no kinks or restrictions. Use free flowing fittings on the manifolds, like the old style merc bronze ones with a big sweeping elbow. Avoid hardware store stuff, unless you smooth out the insides of it with a die grinder.
Try to make both manifolds run close to the same temp. sometimes due to casting differences one side may run hot due to the other side flowing more water. I've put ty wraps on the side that was running cooler to restrict flow and evened it out.
Don't rely solely on the temp sender in the intake.
I recommend bying a hanheld laser thermometer. $40 from harbor frieght. This gives you a better idea of how to pinpoint where problems or restrictions are.
I like to cut the hose in the transom and block it off in the boat and install 1- 1/4" thru Hull, strainer and engine driven raw water pump. Ya can't beat it. Although a buddy of mine swears by the low water pick up Bravos.(still needs a pump)
If drive water pumps were so great we would see them in #6's and Konrads. The biggest reason why I dislike outboards. BBB