With 8/4 stock you should be good to seven feet
The few I have made are normally six foot but I have had customers order eight foot the two where built with 8/4 rough stock. So after planing they are under two inch I would gess about 3/16 under.
If to test put a board between saw benches and see. You will find its guite stiff if you want to lighten the look plane the long side thinner by 1/16 to 3/32 this will change the look of the top guite a bit you only thin about. Six inches in a taper to the finished edge it will not be noticeable
Frankj
In South Jersey the good part of New Jersey, USA.