(1) put the OE iron housing back on & retest it with no stat (no stat wont hurt you for this quick test) & see if it cools. if so then (2) put the stat back into this iron housing & if it still cools OK then the alum housing is the problem. if it still runs hot in (1) then the alum housing and or present stat ain't the problem. if it cools with the iron housing/no stat then runs hot with the stat installed in the iron housing then obviously the stat is the prob. On a side note drilling a 1/16 hole in the stat flat lets you top off pretty much completely on the initial fill and elims hot spots/saves time. EDIT if it ain't the housing or stat & the mixture/timing is in the ballpark with no vac leaks then "at speed" above ~40 MPH it is a rad flow issue (partly restricted)