Just wondering if anyone knows of a way to fasten a traveller system to an older style square hoop with a thinner bar at the top. I have bought the boat (3595) with a kind of double block made from sheaves and tuffnall. Any advice would be great. At the moment I am thinking of fashioning a better version of what's already there.

Thanks in advance

If its not broken why change it? the car and track used on the modern ones are going to coast circa £130 for the track and car or £300 plus if you buy as a complete bolt in unit with track and car already on the hoop.
I asked the same question when I got my Winder Mk1 and was told Winders can make a new version of the tuffnel set up for you for not much money, it would be intresting to know if that is true.
As mine was not broke I have not bothered to change it, it is also worth noting that my first Harken traveler car on a wooden tales failed under load, it pulled the screw holding the P clip out, it was of course replaced under warranty without quibble, still failed in a way the tufnell set up hasnt though!

I have the same on 3587, I think the tufnol malarkey gives more travel for a given width of hoop, I suspect the new hoops are wider at the top?
I have considered changing the rod at the top of the hoop for a harken track, I got a new-old-stock Harken car from ebay, but I have a lot more important changes to make, sorting out the one-string system.
The current system works OK, but it does feel a bit crude and noisy compared to the track on my previous boat.
If I get around to changing it, I think I could move the 'feet' of the hoop to the inboard sides of their fixings to make the top wider?

Ring up Dave Winder and ask him to send you a new car.

There's nothing wrong with the old hoop system at all provided the car sheaves are ok and it's regularly pro-lubed. 

The hoops themselves tend tp break where the bar meets the upright, but this is an easy fix for a welder and costs about £15.