No resale exemption certificate and the six-month rule

We are a dealer that recently purchased a taxable truck chassis for resale. However, when we purchased the truck chassis, we did not provide the seller with a resale exemption certificate, so the seller was responsible for paying Internal Revenue Service (IRS) the Section 4051 tax (FET) on the sale. Is it correct that when we resell the truck chassis, there will be no Section 4051 tax? Does it matter if, prior to resale, we install minor equipment on the chassis to facilitate cargo loading/unloading from the truck (once it’s complete)?