I suggest this tool: Reduced Shank .750 Variable flute X .734 shank X 6.0 long MariTool
I've used the same to go 5" deep in 6061 with great finishes. Obviously you'll be roughing with something else (indexable endmill would be great for that). Leave about .015" on the walls, max RPM and around .0025" IPT with a .750-1.000" stepdown and you'll be pleased. No chatter whatsoever with those parameters on my VF-2ss.
Yes I know what coating it has on it. Yes I know it's a 4 flute designed for steels. It's one of the few I've found that are reasonably priced and have the shank undersized by just a tiny bit...not a full 1/16" or 1/8". This will be way more rigid than a 1" endmill with full flute length.
One hint though: Make sure to check runout. Any runout at all with that much hanging out of a collet is going to ruin your day.