Just bolt it on correct, i.e. bolt the aluminium to the steel base? (as there's no special MIG wire you can use to weld Aluminium to Steel from what I'm reading?) Maybe a dumb question but just checking...
Yes, bolt it. But get a fabricater to look over what you are doing, it sounds like you are inexperienced in metal work, so a more practiced eye would help.
There is a way to weld ally to steel, but it is way out of the amateur league and involves explosives
KF