Need tips and advice for milling slots in thin aluminum extrusions... That are 83in.

So, my company recently got in a job to modify some aluminum extrusions. The print we have is an older one that seems to be missing some dimensions, so I can't really determine the rigidity or depth of the slots until I actually get the material in. Boss wants a quick turnaround as usual and won't buy me much in the way of tooling other than a new drill chuck (Ours is damaged and out of round) and an endmill. I don't have any workholding other than table clamps and a 4 inch vise. That being said, this extrusion is 82.5 in long. I've made and ground some 8*2*1 steel bar to raise the extrusion off of the table, but I've got no idea how to hold this without marrring the part, let alone laying out 82 inches of material when I don't have any long precision measuring equipment. Longest thing I've got is a 12in pair of calipers but I have some slots in this that are longer than the table travel on my mill. My best idea was to indicate the part flat (even though I was told this isn't necessary), and edge find and work from both sides, just edge finding and measuring as I go. Are there any good ways to reduce chatter in the long part or properly support it while machining? Boss has quoted 8hrs of milling time for each of these parts, so I'd be doing one a day.

Boss quoted, ask him how he planned to make it....

Not enough information for a meaningful answer. When I need to slide a part, I set my stops and slide the part. Dowel pins in the T slots, etc. Your steel plate, add a couple dowel pins and use the plate- whatever. It's just sliding the part.

Shouldn't take anything close to 8 hours to put an 83" slot in a piece of extrusion...