Not so much "unsafe" as it is less resilliant to side impact.. which is why saftey consious rodders "box" the frame rails.
My personal position on the subject of saftey in an old bolt is to drive at the speed the vehicle was intended and built to go, and within the limits of the design.
For instance, lets take a 1950 1/2 ton pickup and have a 2002 Dodge Ram 3/4 ton truck 't-bone' it on the driver side at about 45MPH.
No crumple zones, nothing between you an the oncomming truck but that thin little door and a c-channel frame.
You do NOT wanna be strapped in place in the drivers seat in that sort of impact.. because you'll be dead. Getting flung across the bench seat onto the passenger side is gunna hurt, but better than getting held in place while your crushed.
Same truck.. head on collision.. you have a driver in oncomming traffic veer left of center and hit you from about the middle of the grill over to the drivers side headlight. You're doing 50, they're doing 65.
Seatbelt keeps you strapped to the seat, but the bottom portion of the seat slides out from under you, your body drops, and the shoulder strap ends up crushing your throat just before it snaps your neck.. that is, if the steering column dosen't go thru you first.
These old trucks were never designed to be safe.. safety wasn't much of a concern when they were built.. and the flimsy bumpers certianly weren't designed to withstand or absorb any significant impact.
Im saying, in my opinion, that I can think of no senerio where a seatbelt alone in an old truck will play a significant role in saving your life.. and in certian senerios, like side impacts, it might actually REDUCE your chance of survival.
Cabs in these trucks aren't the strongest, either.. in a situation where the truck were to roll more than once, i'd guess the roof would get mashed down like somebody stepping on an aluminum can.. in that senerio, I probably wouldn't wanna be strapped in an upright position, either.. especially not if you consider the giant shards of laminated glass you'd have flying around.
See if you can find some accident scene photos from the 40's and early 50's.. it's nasty stuff.. and I don't think just slapping a seatbelt in would help a whole lot, since again.. a seatbelt is just one component of an entire system of occupant safety.
I'm not saying don't put a seatbelt in your truck.. i'm just saying don't let it lull you into a false sence of security.
