Marlin - sorry you aren't authorized to ask "Quick questions". That terminology is restriced to use of Bill J out in VA.
However, the quick answer is approx. 28", should be the same for a thrower or a blade. (Now I sure hope no one starts asking dead questions)
Jeff - (under edit to your comment above) I think they really are the same. So now we'll have to get a factory expert to identify the correct lengths.
Marlin - since you're making it, might just as well make 3, one 28", one 29" and one 30" - or you could make one with a turnbuckle that allows you to adjust between 27-29"
Jeff - (under double edit) now ya went and done it. "Hydro Harry could be WRONG". For blades the parts book shows a break (difference), IH 548589R1 for tractors serial no. 665,000 and below, and IH140090C1 for 665,001 and above. For throwers it's IH548189R1. Strange, but I'm pretty certain 28" is about the best overall, and 29" may work ok - how about that?