Yes, just use two of the holes. It shouldn't matter if the others are left open... the threads should not extend into the water area. So, it will be ok if they are left open. Yes, cast parts do rust quickly when exposed to water. I believe thats part if boating. When i had my thermostat housing off, i glass bead blasted inside and gave it a good coat of rustolium... to help slow the rust process. Put your old housing on the intake and turn it where it needs to be and put the bolts in... shouldn't be a big deal.
Or if you can't get her to work... i will take off my cast 2bbl mainifold and mercarb and swap yaQUOTE :this therm houseing was a pain ,for these yrs 1990,its in 2 parts it sits up high ,the carb is angled downward,i even had problems with the fuel line hitting the houseing,i had to buy a univ,line to work,i dont want these same problems with this new block, i will try anyway see what happens,thanks rc