I've TIG'd and MIG'd a lot of alum. plate and structural, but never cast. You've got the key: absolute bright cleanliness. I know cast welds up fine, 'cause my top 'o line prop shop TIGs cast alum props and damaged gear hsgs. If your holes aren't too numerous, you can drill them thru with a drill just large enough to clean them up. Use a new angle grinder brush that has never been use on anything but alum, and brush the surface over the drilled holes until absolutely clean. I think acid is unnecessary if you get a bright, native cast surface.
Plug weld the drilled holes with the welder. I'd recommend a MIG with a spool gun with alum. wire, and argon of course. I've used TIG, and it's a real challenge if you're new at it. Practice on a very well shined up thick surface, like the outside of the gimbal plate bolt-up flange, or a shined up piece of 1/4" min alum. scrap. Sneak up on the heat 'till you get just a little - 1/8 - 3/16" penetration. The last thing you want to do is burn thru the wall of the tube. You just need a small plug bead head. There is very low pressure in the exhaust.
Also heating up the first weld area with a propane torch helps getting good penetration. Start on one end of your weld series. The welder's heat spreads very fast, and will get successive welds preheated enough.
