you could try tape and a forstner bit instead of a spade bit, less trauma to the wood but a bit more work to get the hole in the right spot with an offset washer. check out the mar-k instructions for a better description.
http://www.mar-k.com/_assets/images/...s_3-8_ofst.pdf
or here for pics and description. older truck series but same methodology
https://talk.classicparts.com/thread...ed-wood.12180/
the offset washers are used for the bed to frame bolts so the washer jams in the hole it is recessed into as the bolt is turned. the bolt has a back like a carriage bolt has and the washer has a square hole to accept the bolt. the washer is set down into a shallow hole in the wood, when you tighten the nut up under the floor the bolt can't turn because the washer jams in the offset hole. I think this is how it works anyway.