I put 2003 silverado seats in my 63 without much modification. I drilled the holes and on the bottom of the cab I used some washers that were about 2 1/2 inches in diameter. Its larger than the original mount plate on bottom. The seats have been in for about a year. The seats I have are buckets and Im just now getting around to make a bracket for the center seat. Hope this was useful.
Bumper
