BLUF - Another vote for COSTCO.
I started buying vehicles in the 1970s and have ALWAYS ordered them. I have never bought off the lot. I've never had a problem with this approach.
I would not budge. Stick to getting a truck with only the options you want.
When I bought my 2013 F350 I tried everything - credit union preferred dealers, buyers agents (this has worked for me in the past), contacted the dealership's 'internet salesman', sent out emails to all the local dealers - but did not get very good replies or quotes.
So in desperation I decided to try Costco. I followed their instructions and was surprised that the price I got was the best. So that's the route I went. I emailed my option list to the salesman and after getting the quote, I visited the dealership, talked with the Costco POC salesman, and ordered my truck. The salesman told me that 90% of his orders are due to Costco (This was at a large dealership in Florida). The truck was delivered about 6-8 weeks later. When I picked it up I asked about the pricing. The salesman said he has to price 'aggressively' when he prices up a vehicle for Costco members.