LOL. This would be a good time to see if a 500ml bottle of valbazen will treat up to 400 birds....better buy two bottles. You can order it from Jefferspet.com.
Currently all wormers have a withdrawal period. If you believe that you or a family member might be susceptible to a reaction from residue in eggs, toss them in the garbage for two weeks after the last dosing. Dont sell nor give away eggs during that period. We eat the eggs after using Valbazen, I'm still here typing.
Administer 1/2cc orally to each chicken, redose in 10 days to kill hatched larva missed during the first dosing.
Worming up to 400 birds might take 2 or 3 days to worm each one since your birds free range. I hope you have someone available to hold a bird for you while you dose them as well as one or two other people to help corral them into a pen.
An easy way to worm that many birds is to worm by breed if possible, different breeds each day.
Or, If they are all one breed or even different breeds, leg bands work and color wouldnt matter to signify a group has been wormed.
Then when you redose in 10 days, simply remove the leg bands. That way you'll know what birds got wormed.
I've helped worm just over 100 birds and it took only about an hour to worm all of them. Of course everything was pre-staged and all the birds were penned. There were 5 of us doing it at the time. I was doing the worming, one person holding the bird and the rest catching them in the pens. It was like an assembly line.