May sound like a dumb question bit here goes. Is it possible to run a 68/83 hollowtech crankset on a frame designed for 68/73? I can see the crankset fitting without issue as the crank axle is the correct width but the issue would be chain line after fitting. I know FSA produce crank axle spacers for my crankset (gravity MegaExo) but I can't see this helping chain line other then stopping the crankset having lateral movement. My build is a 2009 kona four deluxe with a 1x8 drive train currently setup with sram x4...but I've broken my crankset so if possible I'd like to use the one I have (gravity MegaExo 83mm) rather then fork out more money at the moment. Is there a way I can do this?
I'm assuming the crank you have is for an 83mm shell, i.e. it has a longer spindle than the common external bearing cranks for 68mm or 73mm (68/73) shells.
It sounds like you know this, but you can't space the cups this much and still have okay thread engagement. You could put some spacers there, usually up to 2.5mm per side, or on some BBs 5mm drive side/2.5 non-drive.
You could use spindle spacers to take up all the slack, and by making the cranks asymmetrical in the frame you could make the chainline correct, or correct-ish. This raises the question of extra leverage on the spindle since it won't be supported where it's designed to be, especially if all the spacers are on one side. That becomes an at your own risk thing. I suspect if it's a pretty burly spindle it would be fine for xc/trail type riding, but that's just a guess.
Since you're 1x you could also employ chainring spacers to help a little with chainline, maybe up to 5mm worth. (I actually don't know what the line is there before there are consequences. They come up to 3.8mm when designated for mid/outer position use. Use steel chainring bolts.)