[Keep on Trucking]

IMO don't think is going to be a problem. In Tx we don't have to show registration as its on the windshield. Customizing your vehicle like this should be fine. If you were turning it into a limo and there were some law that governed length/width...ect then you would need to register it that way.
If you tell the lady behind the counter what you did then you can bet you will be in for some more costs and red tape. Just go pay your reg fee and keep the pie hole zipped.
Again IMO.