They're mainly wanting to see pictures of the vehicle's current condition. If you add mods (and thus, value) then you should be able to call them back, make the adjustment, and pay whatever premium delta would exist.
FWIW, I had also tried setting up a policy with ACI byway of USAA. I ended up going with Hagerty and would make the same recommendation. I found Hagerty was easier to work with, they're glowingly recommended by all of my enthusiast car friends, and they'll take pretty much any car that lives in a garage. ACI, to me, was not as accommodating, didn't include a lot of the perks that Hagerty would, and they simply weren't willing to insure two of my vehicles. They would take my '86 El Camino, but not my restored 1991 560SEL or my 1-of-120 50k mile SC300.
There should be some carriers in your state that will specifically cover off-road vehicles. I would bet that both ACI and Hagerty would rather you not do trail driving. There's a local group near me called Country Financial that offers agreed-upon values and does not penalize for off-road damage. I was going to have my 100 written up with them for $60k but some admin issues (my fault, not theirs) prevented that.