How neat of a job do you need to make? A Stihl cut off saw with a 12 inch blade will typically cut a little over 4 inches deep (because the arm and axle the blade mounts to occupies the center 4 inches of the blade diameter) so you'll have to flip your blocks to cut 8 inches and marrying the cuts up is really hard without a frame but if youre not too precious about the neatness because the cut isn't on show it may not matter. Larger stihl saw models that take a 14 inch blade will cut 6"
Floor saws are a variation on the stihl saw, they look more like a sack trolley carrying a saw and engine and can have enormous blades. You might need bigger than 18 inches blade to cut 8 inch but 6 inch will be attainable with an 18 inch model. They aren't very portable/agile though but workable if you can lay your material on the floor and put something around it to help flatten out the way for when you run the floor saw on it
You can get (probably hire) a tool called a ring saw which is like a Stihl saw but the blade is a ring that runs around a large plate the same thickness as the blade. It is driven by rollers at the edge so nearly the entire depth of the blade can be used for a cut. They're petrol or hydraulic; if hiring a hydraulic one factor in the cost of hiring a power pack too
It's also possible to obtain chainsaws that cut stone; they have a great depth of cut but don't make for a very decorative finish as they're hard to cut straight with. The blades are also eyewateringly expensive and wear very quickly compared to circular saw blades
I've successfully modified one of these:
https://www.ebay.co.uk/itm/195874394778 to carry an 18 inch floor saw blade for an 8 inch depth of cut but I have also created a frame for it so it's held in place and does its cutting like a compound saw would. I'd hesitate to pick it up and wield it like a stihl saw, with an 18 inch blade in. You can buy similar, eg bridge saws and table saws that have 18+ inch blades but definitely more a commercial than DIY endeavour
All in, without further details on exactly what you're cutting, and how you'll use the result it's hard to make a firm recommendation but I think probably the ring saw is your best bet unless you're setting up in the stone cutting business