Depends how BIG you are!! and how long your arms are for the front ones. let grain pan down at the rear onto a barrel or something solid and crawl in there and git her dun!!
You can crawl up on top of the pan if you are very careful not to damage the sieve under the pan. it is a pain to get up to the front and if you have an air gun with an air ratchet i would recomend that method of getting them because it is much easier to hold it in place than to try to fiddle with a ratchet.