Does anyone know which of the Trichocereus would have the lagest girth? All the references i can find give the hight but not the girth, i'm thinking grafting stocks here.

In relation to grafting stocks all trichos will grow more than large enough for most scions. What are you thinking of grating ?

T. tersheckii and T. pascana are quite massive. T. pascana has a diameter of 30cm on a fully mature plant I think tersheckii is similar, maybe a bit smaller.

Not sure these are the right trichocereus' for grafting, actually I wonder how/if it would work. You could make a massive graft on a plant with a diameter of 30cms! But i think T. pachanoi, with a diameter of 5-10cms would be a better bet, as these a easy to find and grow much faster.

Micro

I'm thinking Fero's.
Around here they grow pretty well in ground untill their root reach the water table & then they rot, usually when they are just becoming a nice big mature plant. so i'm thinking grafting simaliar sized fero's on to Trich's hopefully they are compatable & the Trich's can match the girth of the Fero's so the dont have a weak spot when it gets windy. I've allready had my Browningia snap off at the graft point

This sounds like fun to me. Grafting huge scions to huge stocks Any tricho will be fine just make sure you get a nice say 1m long arm from an old plant and the diameter should be 10ish cm and could easily take the weight of the ferro. Also make the graft on the younger material as the older woody stuff will rejct the gaft.

Post some pics if you do it, would be an interesting project. Hmm I have so many ideas flying around my head about this....You could also graft it to the tricho before planting the cutting this would make it easier to graft and then you simply root the cutting in a good position in the yard.

except for one big old gracilis most of my Ferro's are 4 to 6" diameter atm but since the get pretty massive i think the scion should have an equily massive stock, plus it might push them along a bit.
If i can find enough pascana cuttings I'll start grafting in mid spring, thanks for the info Micro.
When is wood too old for grafting? when its starting to form bark?

In another post that i can't seem to find now there was an idea to graft a carnegia onto a faster growing stock and it was recommended that a thick Stenocereus would be a good choice for a huge scion.

here we go:
http://forum.auscactiforum.net/specialist-cultivation-f31/stocks-vs-stocks-t58-15.htm#3061
Kada says that Stenocereus are super fast growers and 4-5inches across.
maybe this would be a good alternative.

Toppling either from wind or racing possums is my bigest concern, I've allready lost my Browningia & my nicest Pilocereus to the racing possum. The graft point seems to be a week point on the plant once they start getting a bit of hight & leverage, perhaps some "feature" star pickets could be in order.

To answer an above question by shortly. Once the grafting material (of the stock) is over a year old I feel it's too old. It should be far far from woody, the younger the better the chance of sucess. Plant horomones are pushed to the growing tips and hence are better for grafting to.