Sorry mate, I am not having a go at you, you have enough on your plate!
You said you were going to be honest if you sold and then said 400+ BHP, but you have no proof other than you almost caught an FQ400, that would not cut it for an experienced buyer, it may get it sold to some poor unsuspecting sod who would end up having his/her pants pulled down on here whan they came showing it off, and thats how you eneded up with it right, seller telling you he had a dyno sheet somewhere, and it goes missing, you never loose a dyno sheet IMO if it has good results !
You may find it does have 400+, but TBH it seems to have so many unknowns I doubt it and until you have the spec I would not test it on a dyno