I was told last night at the Barrett Party I was at, that the US Gov't had ordered everything destroyed that dealt with the assisnation (i.e cars, clothes, ambulance etc.).
I remember reading something (no idea where, sorry) that the Lincoln convertable he was riding in when shot, was retro-fitted with a roof and bullit-proofing and used by Johnson (maybe Nixon, too?) before being retired. It seems that if anything should have gone in a museum and not altered... I don't know, maybe it is part of a cover-up, maybe it was a way to not be reminded of a "national tragedy"?