Need help finding correct belt size

[bertsmobile1]

Its a 6 speed. When following your link i cant find a combination of 400 frame and 6 speed, so it should be a 600 frame I guess.

Im still wondering how the vari belts behave when moving the shifter all down, the pulley slides forward so that it makes it easier to change the front belt, but i really cant seem to wrap my head around that it can move all the way bac again with the belt fitted.. The belt isnt stretchy enough to allow for such a big movement.
Is there a order to replace the two belts? the rear one or the front one first?

The top belt is pulled back by a farily hefty spring .
When you put you foot on the go pedal you work against that spring .
The double pulley slides up & down which changes the diameter of the pulley so one gets bigger & the other gets smaller.
It is in effect the same as a set of stepped pulleys you see on small drill presses.

 

[Morbus]

So the correct belt arrived, and i installed it. Drove a test round, without the knifedeck installed. Worked fine for a few hundred meters, then the rear/upper belt jumped off. Maybe i got it routed the wrong way around the tensioner pulley. So reattached it, but same thing happens. Then tried routing it the other way around the tensioner, also moved the spring to the tensioner pulley further out, so it stretches a bit more (=tighter belt?)
Still jumps off after a few meters now.

What can I have done wrong so that this happens? The bracket that holds the dual pulley shaft was taken loose, but reinstalled in the same way..

 

[bertsmobile1]

This usually happens because the transmission support bracket is loose which allows the big pulley to tip down at the front .