Hello charlie tuna -- Hope you're experiencing the Forums for all they're worth. Keep asking questions like this!
GO Bob & overlander64 have given you some good info. I've seen these at a local SOB RV dealer for about $60 a pair as GO Bob suggests. Locally I'd think you can save significant shipping.
I'd welcome better input from members but these are installed with approx 1/4" lag screws -- galvanized would be great. Removing old B.A.L. jacks may cause stuck mounting screws to twist off and I would not recommend using that same hole -- grind off any sticking out parts while keeping the bellywrap as intact as possible, then inject a little Vulkem on the old hole. (Me? I prefer Sikaflex 221
for most applications. The grey is a close match for aluminum. NO SILICONE CAULKS! Keep unused tubes of caulk in your home freezer between uses -- makes it good for a very long time!!!
You'll probably want to offset the new holes 1/4" to 3/8" or so from the old -- the mounting holes in the base of the B.A.L. jack will give you an idea of the layout. You might make a template of posterboard unless you're up to jockeying with the jack while trying to measure & mark. You'd want very sharp (new) twist drills to drill the holes (electric hand drill) and would want to stay a bit undersized. You only want to mount these into frame members
-- so get under your Airstream and check out the rivet line headed crossways on the belly wrap. Run an electric drill at low speed but get good pressure pushing up. Buy your screws first and estimate or measure shaft diameter to determine what size twist drills to buy (approx shaft diameter + a whisker but not at all close to the full thread diameter). You don't have to tap the new holes in your frame -- a hand operated socket set has been successful on my end of the world.
[on edit: I have a general recall that lag screws were used for my Argosy. I just pulled a loose screw from one of my Safari BAL jacks. This is a 1" long 1/4" hex head machine screw (1/2" socket head), therefore is a more gradual sloped thread and theoretically tighter holding than a lag screw. Got enough info?]