If your main objective is to have the 1/2" square hole parallel with the bottom of the shank, they did make straight, right hand and left hand holders that way. The common forged holders like 4-L have back-angled holes for HSS bits ground in such a way as to maximize regrinds. When brazed tungsten carbide bits with the carbide set at angles became popular, Armstrong and others made forged holders with a T prefix that suited those tungsten carbide bits.
So, search for Armstrong holders with models T-4 S. T-4 R or T-4 L. I bet they are scarce, but it would not hurt to look.
I did find one: ARMSTRONG NO T4-R ABOUT 1/2" SQUARE HOLE LATHE TOOL BIT HOLDER SOUTHBEND MONARCH | eBay
As for the bending method, an oxy-acetylene torch can heat a local area behind the rear of the square hole so the bend is concentrated in an area that does not distort the square hole or most of the shank.
As to heat treating, you could ask a pro heat treat shop if you are not experienced enough to do it yourself. In general, alloy steels like oil quench and plain carbon steels like water. One way to sort it is to try oil first and do it over with water if it does not get hard with oil. But the pro shop is your best bet. Heat treat is pretty cheap and doing several parts at once spreads the cost. Part of the cost is the shop figuring out what steel you have, of course.
I think I have some of these along with some in #5 size. I'd have to go look to see what I have.