Exactly what roverp480 said.

When I registered my restored (and re-framed) 1971, it was not yet 25 years old at that time. I had to jump through similar hoops- the VIN not having enough digits for their liking etc., but no mention of the VIN on the frame (or not).

The kicker was, after a signed letter from the prior owner stating he didn't own it (no title available) I had to get a "state assigned ID number" for the vehicle. Went to do that, and the "assigned" number was whatever I wanted it to be... (still amazed by that) so I chose the VIN number, which was added to a sticker via typewriter and then stuck on my door jam. $50 and done.

That truck is now gone, but the bottom line is that the DMV employees are not familiar with their own procedures, nor the DOT exemptions that come along with vehicles over 25 years old. In your case, with an "antique" vehicle, you don't have to comply with the laws in place in 2023, but you do have to comply with the laws in place in 1973.

In this age of litigation, I'm surprised there are not a flock of lawyers assisting these procedures...