9/10 wt fly blank?

Im planning on getting a 11' fly rod blank 9/10 wt and making a spinning rod. how heavy of lure can i throw and what would be the line rating?

Sounds pretty ghey....for springer's?

Why not get a spin blank for the weight you want to cast?

It will be much lighter than you think.
I learned this the hard way, after building a rod out of a 7 wt. Fly rods have nearly all of their backbone in the last 10-14 inches of the butt. That puts it all inside the handle behind the reel of a spin or casting rod.
The blank you described should make a nice light/medium side drift or float rod.

Buy a center pin rod

Back in the day... mid 80's I think...we thought we were hot stuff building jig rods from 10 wt fly blanks in 10'6". They worked very well. Michigan-style handles were nice. This was before float/jig rods were widely available. I still have one, and still use it from time to time and don't feel too handicapped by the relative slowness compared to what's available now.

That said, you could get a better, lighter spinning rod off the shelf for 100$ or so.

If your intent on the fly blank idea, I never went above 10# mono on mine. 20# Fireline seemed to work about right also. Handled up to a half ounce or so nicely. Caught a few on spinners and drift gear with it also. With a decent blank you should have a nice rod.