No ,as long as you are sure the kit makes the 51% rule. All RV kits are now on the list of approved kits. MD-RA probably would like for you to have the letter though but I cannot see a problem.
 
I think technically they want you to have the letter in place before commencing work on the project. I don?t see how they would object to receiving the kit before getting the letter in. You will need a serial number, your Vans builder number works.

The letter itself is simple enough, but this the point you declare your plane?s gross weight. Changing it later is difficult.