So, I got back home from lakeside on the evening of 11th and just had to open the box and see what I got. The cage with all documents (including shipping docs indicating that it had been shipped on 29th of May) were there. On the next morning I started fitting the cage in. The rear part was a clean fit and attachment plates which are to be welded chassis were easy to locate. Front section was slightly more difficult... I managed to mark approximate area for the plates there and cleaned those sections of floor for welding.
My neighbour has a friend thatt has been involved in building race (mostly rally) cars for 30 years and he's also a pro welder so he arrived in the evening with all equipment to my place. Rear plates were welded in quickly but just after midnight I called it a day. Something was wrong with front section and it was a time for thinking and planning. Pro's woords were somehow comforting when he said that he hasn't yet seen a cage that has just been "thrown in". I twisted and turned the front section in and out of the car the whole Wednesday and it repeatedly seemed that although it's nearly there few tubes are too long. Even an excessive use of force dindn't help. After a telephone conference with him I made a decision to start cutting parts.
So on Wednesday evening, after shortening the tube that runs along the upper edge of windshield for 4 centimeters -1cm at a time, fitting and measuring again- I was able to get door hoops' legs fit to lower end of the A-pillar. Before that they were twisted towards each other trying to meet near cardan tunnel... and the upper edge was trying to push out A-pillars. Not to mention the light switch that was completely blocked by a 38mm steel tube. I've heard that having driving lights on in darkness brings added value to driving...
Next issue was to get door bars and the one running along the dashboard, connecting door hoops together fit in. Now I needed something else than an angle grinder, so I went to a friend's shop and he shortened those bars in a lathe. Door bars needed 3 cm's cut and the dashboard bar 8 (yes, eight!) cm's cut. Fortunately there were enough threads inside the bars so no new threads were needed. Bar ends are adjustable, very neat items indeed, and thus the cage can be pre-loaded.
This adjustability comes handy in fitting too and after this operation I could finally push and pull door hoops' ends to correct places. They were welded on Friday morning. Immediately after that I cleaned up the welds and painted the plates. Friday evening the whole cage was finally in. saturday was spent with my wife and on Sunday I lifted the car up, drilled last holes for four bolts that comes through the floor, replaced burned sealant and started installing the interior.
Pics are from Monday evening, the other seat is still missing and some finishing yet to be done because I had to go back to work.

In general I'm pleased with the quality of Heigo's cage. Paint was flawless, adjustability with those threaded bars is good and their fork ends look cool. I have to admit that although their customer service is below any standards. That fitment issue I had was mostly inexperience. If I'd ever install another one I'd know right away what to do, now I had to ask help and get confirmation before starting cutting.

What's interesting with this cage is that it is slightly wider and longer than GT3's OEM cage. I have GT3 CS's floor mats and while the part between rear struts fits perfectly the holes on the floor doesn't. Main hoop (B-pillar) is wider, those inner door sill plastic liners must be cut. At front too. And at front the cage goes more forward, there's more room to twist shoes in.

 

good info. Hard to weld to the body. you need to grind off the galvanizing.