I am building a striking anvil with a piece of mild steel that is 2"x5"x24" ,  I might be able to get some one inch plate from work and I have 3 inch tubing.  All the pictures I have seen has 2" tubing splayed 8 degrees out with three legs.  I am trying to do it without buying anything,  3inch tubing won't work with the normal configuration but what about a t base with two uprights?

 I have heard that you want the legs to be at 83 degrees or else they will start to flex, correct me If I am wrong. another thing that I have heard, maybe Brian Brazeal came up with this, I cant remember, but is that you can fill the legs with a mixture of sand and oil to dampen the noise. I might not work with only 2 upright unless you had it mounted on a cement slab in the ground so that it doesn't fall over or weld a steel plate on it, but than again I might be wrong, wouldn't be the first time.

                                                                                                    Littleblacksmith

I was thinking of a base shaped like a T,  with the two uprights coming up from that.  I may have not been very clear.

Here are a couple pics of Brian B's set up or was 3 yrs. ago, not great pics but the basic design  thought I had a better one than these?  3 legged I remember. 

If you have a flat floor to set it on or bury it in, two legs coming straight up will work.  The width of the footprint is important because it stops wobbling as you strike the metal.  The wider the footprint, the less wobbling.  With an inverted-T design, you'd likely get some wobble unless you built the base plate to be really wide and gusseted so the plate couldn't flex.

You're 3" tubing will work with the tripod (normal) configuration.  All you need to do is cut a small fishmouth out of the upper few inches and hammer it closed.  This will reduce its size so it all fits within the 5" anvil body.  Since you're welding anyhow, five minutes with a cutting disk on your angle grinder won't add measurable to what you have to weld.  It sure won't hurt the strength of the legs.