Suzuki 2 Strokes

76 GT550. Is it easier to install the box then the plenum or install the plenum to the box and install as an assembled unit?

I would have to say, install the carbs first. Followed by the air box and plenum as one. It's a little easier reaching and tightening the screws as one assembly before you attach it the carbs. With the complete assembly in your hand you can easily tighten the screws. Plus, the plenum is quite flexible getting it and the air box where it needs to be afterward. Have you ever done any carb work on Suzuki GS ? It's a bloody nightmare working with it's airbox. Nothing can be done on those bikes, without moving the airbox first. Which is quite a chore. Horrible design.

Have you ever done any carb work on Suzuki GS ?

Yeah, my GS1000. What a nightmare -- even with 2 people. I've done it 3 times. After the third time, I sold the entire assembly on eBay and stuck on pods. Increased ease of doing carb work by 1000%. In stock form getting them off was a PITA. Putting them back on was a triple PITA. Horrible design indeed.
In retrospect, I wish I hadn't sold the stock assembly. It would have been a plus when I ever sell it