The last 69 impala that I worked on had a floating rear tack rail.This image shows the tack rail bolted in place.This shows the tack rail unbolted, ready for removal.

That last bar measurement is taken from the body opening to the center of the bar.
The top manufacturer supplies that info.
When installing the bow pads, I like to have that rear bow measurement slightly forward by about 1/2" of the dictated distance. This is because the rear curtain/window, when installed, will stretch the bow pads tighter & pull that rear bow back to where it should be.

Hi fred thanks for getting back to me, being from the uk the impala is all new to me, the hood set up is familiar but I'm pretty sure its missing parts!
Are you saying the sides of that one was on a slide bar and a loop in the rear window piece so it can slide up and down? i have no bars or loop in the new window piece if that's the case.

All i have is that rear horse shoe bar which stops say 5 inches after the curve of the body and then nothing to the B pillar/hinge, in the body there's maybe 4 bolt holes that go along and taper down following the body shape so i assume there should be some hockey shaped bars?

Thanks for the reply, yes i understand your 1/2 inch method thanks for the tip!

From the image that you posted, it clearly looks like the tack rail extensions are missing. Are you sure that this was car was not altered from the floating tack rail?

To be honest i don't know this is how it arrived. The old hood has bolt holes where the extensions are so i assume they're missing and whoever took them off has them!. i assume by floating tack rail there's a loop in the bottom of the hood which slides up and down the bar to relive tension?

Could you advise where these missing parts could be purchased?

thank you

@Gareth Judd From what I was told from Hydro-E-Lectric, the rear tack rail is no longer being produced and is not available. I was also unaware that there were two tack rail versions for the 1969 Impala.

The parts you are missing can be easily fabricated to fit the car. If you were near by I would gratefully make them for you. If you cannot acquire them locally, Send me a pattern with the dimensions and I will make a set for you.