So what weight of fly rod gets me anywhere close to the utility of the 9' IM rods we're talking about. THAT is the blank I want to try.
I picked on Fisher, X-long and 6-wt for combined reasons. Fisher rod butts are UPS-truck-stopping.
The progressive fly rod taper gives you the tip you need, including the ability to cast both lighter and heavier lures than the purpose-built fast graphite spin blanks.
If you want that light-tip, going up in line weight will probably sacrifice it, though if you can find an X-long 7-wt or even 8-wt Fisher, you're probably still in the ballpark you want to throw.
You can calculate the max lure weight by converting fly line grain weight to ounces. The light end is all about the tip, and Fisher's progressive tapers are famous for close-in control (light tip - light lures).
You might pm ARReflections - Eric, and ask his observations on his custom rod.