use
dd-wrt.v24-27858_NEWD-2_K2.6_mini-nv64k.bin
or
dd-wrt.v24-27858_NEWD-2_K2.6_mega-nv64k.bin
the dd-wrt.v24-27858_NEWD-2_K2.6_mini-WNDR3400.chk in that directory is to go from stock firmware to dd-wrt.
Since you already have dd-wrt use one of the two builds I listed ... same firmware as my E1200v2 and the WNDR3700v3 uses.
It's a decent build.
EDIT: I recommend you install the mini version
Ahh, the newer 27858 mini build to which you pointed me was the ticket! The WNDR3400v1 began functioning as an access point right after I made the mods called for by the wiki https://www.dd-wrt.com/wiki/index.php/Wireless_Access_Point. After that success I immediately decided to update the other WNDR3400v1 which functions as my router. The configuration appears to work flawlessly. I'll review the other parameters you'd suggested earlier and change accordingly.
Thanks much for your advice and patience in helping me out, mrjcd. So appreciated!