I've never done anything that long, but I'd say as close as you can stand. at least every 2 inches, but don't weld those 2" spaces up all in one go. aluminum moves a bunch when it's hot, so it will close the gap on you hard if you don't tack it off well and take it easy welding it. .050 should cool off pretty quick, so at least you've got that going for you. for sequence, I guess I'd probably tack both ends, then the middle, then keep dividing the spaces in half until i was down to 3" spaces, then start at one end and put tacks in on 1" or a foot, then weld every other one up, then move to the next foot, weld every other, ect. that should keep the alu from moving too much. of course, you're doing a lap joint, which isn't quite as critical. on a butt, if you don't tack it off like crazy (and even if you do, sometimes) the gap will close after the first few inches, then it will pile up like a fault line, which is a royal pain to hammer out.
if you start getting an overall bow working from one side to the other, switch to the other end every other foot. another thing, you can't trust what you're looking at until the aluminum has cooled down. most likely the whole thing will bow out while it's hot a little bit, but when it cools down, it will suck back in. when in doubt, just let it cool off.
if you start getting an overall bow working from one side to the other, switch to the other end every other foot. another thing, you can't trust what you're looking at until the aluminum has cooled down. most likely the whole thing will bow out while it's hot a little bit, but when it cools down, it will suck back in. when in doubt, just let it cool off.