In my 1940's bathroom remodel the old toilet didn't even have a flange.After replacing the rotted floor around the toilet,I have my ceramic tile down and it's flush with the top of the cast iron pipe.My question is what type of flange to use.The cast iron only goes down 1 3/4" before it narrows and turns 90%.I seen some of the plastic flanges with the rubber rings but they are way to long.I thought about using a regular plastic flange and cutting it down to (1 3/4")fit inside.

Not too many plumbers remember when we set toilets without flanges. A flange simply secures the toilet to the floor. Nothing more. It's not a seal and it doesn't hold water. We used woodscrew type closet bolts and set that sucker on a ring of plumbers putty. And they held forever.
You can set the bowl directly over the cast iron pipe and mark the holes. Now take a 1/4" masonry bit and drill the tile on your mark. You may now place plastic inserts in the holes and proceed to screw in the closet bolts.'
Place a wax ring over the horn of the bowl and set it down over the screws. Give the bowl a little twist to seat the ring and tighten the bolts.
However, If you desire a closet flange clck on; www.fwwebb.com/pdf/2003pasco/pas_d-3.pdf -
And scroll down to the brass closet flange(49440). Secure it to the tile using screw type closet bolts and plastic inserts and set the bowl in the usual way. Good luck, Tom