I dont do "alot" of aluminum welding most of the stuff i work with is 1018 or simuliar steel but any time i do have to weld aluminum if the part is around 1/2" thick or thicker i will pre heat it to about 150-175 degress.

Mainly what this does is it just removes any surface moisture from the aluminum so that you get better penetration.

Any experianced welding shop should know the proper procedure.

Also i would remove the valve train from the heads myself so there is no risk of damage.