Before our tech experts begin asking several dimensional questions (and we will need them before we can advise you), I recommend that you take a few minutes and review the following link:
http://www.rbracing-rsr.com/rakeandtrail.html
Gather all the particular dimensions as defined at the 'trail calculator' near the bottom of the page and post those pertinent measurements in a post here. We can go from there to hopefully help you. By the way, your forks are not FLH, but from the FLS line (if a Fatboy, likely an FLSTF). That is significant primarily due to the differences in the fork tube 'offset' and the overall forks length.
Also, in addition to the measurements cited in the webpage, how high is the bottom rail of the frame when it is level measured from the floor?
Undoubtedly you will have a truckload of other technical questions as you build, and this is the absolute best source for knowledgeable and experienced experts anxious to help.
And as I always say,
all advice is guaranteed or your money back!