RV-4 Front Seat Cushion Support
As other posters have alluded to, the plans give no direction about the front seat cushion. Van pretty well leaves that up to you. Having said that, depending on your height, there may be as much as 5" total height there that can be used for that purpose.
I'm a little guy (5'10" on a good day, when I was younger - now, not so much!), so I built a "toolbox" that screws to the floorboard longerons and provides the proper height and "rake" for a suitably sized cushion (which was then attached with Velcro).
Mine happens to go over the spar carry through and "lands" on the top cover of the forward tunnel.
But about any option that meets your needs in that area is up to you.
YMMV - but this is one way to deal with this issue.
HFS
RV-4 S/N 1136