If a guy has a bigger and significant reach advantage, one thing's for sure: you can't box from the outside or he'll mash you eventually; Either you try to make him come to you and while he's "unbalanced" because of the forward movement, you close the distance and you bang, while he's in movement you have a small window to do. Another strategy woud be willing to eat perhaps a punch or two, close the distance and as it has been mentioned before, put relentless pressure and force him to switch from offense to defense. If you can keep him on the heels by constant pressure, he won't have as much power in his fists because he won't have the proper stance to transfer weight. Try to avoid the ropes as much as possible as it is kinda deadly if you're against them and if the guy has a bigger reach, trying to pivot a lot around the middle is something to do as much as possible too.