As one of the earlier comments mentioned, you're so out of luck.
Any 6.4GB drive is going to have to be IDE or newer (like EIDE). Your 286 likely does not support EIDE; references to MFM/RLL drives are spot on.
Even if you could somehow get IDE to work, the IDE controllers of that day did not support drives that were 6.4GB. Hard drive limits mentions at least 8 barriers that existed between 20 MB and 6 GB. You're bound to bump into those. Even if you could somehow figure out how to get a drive to physically connect, your 286 is bound to have a small amount of RAM, etc. You could have a 486 and have troubles getting over 500MB, which might be solvable by using a DDO or, better yet, replacing a chip. But a 286 with 6.4GB? That's just too insane. I can appreciate the "hack value" of doing something unique and challenging, but you've got to aim lower. The technology gap you're proposing is just way too big.
Note that I'm not even talking about using 6.4GB of data. I'm saying you can't use the 6.4GB drive, even if you were thinking of only formatting a 40MB partition.
For upgrading the disk space on a 286, offhand my best guess for your best bet would be to see if you can use a parallel port ZIP drive (100 MB) or SyQuest EZ-Drive (135 MB). Or use a network card to access files remotely, though that may be a substantial challenge (probably easier than getting a 6.4GB drive read on that old hardware, though). I dunno if it might be faster, though: parallel ports weren't known to be a super fast way of accessing disks, but then again, MFM/RLL drives weren't known for their speed. MFM/RLL drives also weren't known for their reliability