Edit, never mind, I'm sure you will do a good job on it.

I attached the retracting side of the belts to the bottom of the pillars between front and back seat, not to the floor.  Tight, but enough room.

Fender washers are for shimming fenders, not betting your life on.  Go to Juliano's Rod Shop and look at their install videos, and their anchor point reinforcments. 

What ever you do, DO NOT ATTACH belts to the frame.  You and yours want to stay with the body, not the frame should they part company.

Also careful you don't drill into your tail pipe or muffler.  :)

Edit, should add mine is 52 Cranbrook 4 door.  Should match yours.

I installed three point seat belts in front and back.  Get the kind where the latch belt side of the assembly is round, (solid material) that sticks straight up between the seats.  I had to file a opening for them to pass through that sheet metal, then ran them between upper and lower seats.  If you use the lap belt style then you are good to go after that.  Use large fender washers under the floor.
If you go with three point style, take off the pillar headliner material as one piece, if will come off like a door panel does, and you will see a opening up high where you can anchor the third point. with a bolt.  

I use to have pictures of it,  if I can find them I will post.

It's hard to believe, but my  '51 Cranbrook has gone its entire life without seat belts.  Sixty years later, that needs to change. 

1.  Starting with the front seat, an initial inspection suggests that the front seat is formed by a solid steel pan, making it impossible to simply pass the lap belts between the seat cushions, without first performing some form of surgery on the seat pan.  If so, is there a recommended procedure?

2. Then there is the hole drilling issue for mounting bolts.  Before I get out the big bits and start drilling, are there any pitfalls to be avoided that may not immediately meet the eye?

3. Then there's the back seat, but let me get the front seat belts in first, please.

Thanks for any advice you can offer!  KranbrookKenny