My experience won't help much, but I thought I'd add it in here anyway. Partly just to show that what others are doing has little to do with what you should do. I use 3 X 60 and 3-1/2 X 120 almost exclusively 36 grit.
You call that a limerick?
A Limerick is more like: There was a young lad name Dempsey
Who had tired of programming CNC
So he went to the town
Where the big blacksmiths pound
And cried "I wanna be just like them, see"?
Yeah, but when was the last time you did something where that would cause a problem? Seems like a problem everyone worries about, but never (OK, rarely) runs into.
Around here we've run lots of different hammers right on the floor with a timber pad underneath. I think one of them (in the Craig's list ad) was a straight helve (un-guided). I'd be happy with any of them.
According to the Energy Information Agency, the current average price for propane is $2.82/gal. in bulk. You sure you're not talking about a 100 POUND cylinder? About 4 feet tall and 18" diameter?
You've got a very nice control system there Ken. Is the control 100% with the motion valve? Or does the treadle also control some sort of air throttle too? When small steam hammers have a foot treadle it usually only controls the throttle. Not a very good system most of the time. Have you seen the video of the steam hammer that I modified to control both valves with the treadle?
One reason (and there are others) I never come down straight from the top is that when you shut the forge off they act as chimneys and all the hot gasses go straight up cooking everything.
No, it's got a short angle in and a long angle away from the venturi. Here's some pictures of one running in a forge I banged together in five minutes with some fire brick. Not the most efficient way to build a forge. Note that even with it closed up pretty tight there is a minimum of dragon breath. A true venturi should have no trouble pulling in enough air.
Of course there are degrees of hard and soft. Cast iron is too soft for an anvil and pure wrought is too soft for a hammer. In the old days they went to a lot of trouble to put a steel face on a wrought iron hammer (same with anvils for that matter). Basher, I think your soft hammer is hard enough. Unhardened is still harder than soft material. Hard and soft are just too general. How hard is hard? How soft is soft?