I haven't actually touched Brace. The reason the Bofors and Mortar Pit have less health in Brace after my changes is because Brace's main effect is to reduce incoming damage by 25%, not give the emplacements more health. I'm sorry if you already knew this, just making sure we're on the same page. 
The Bofors costs only 10 manpower more than an M5 Halftrack and is significantly more durable and dangerous. I think some reduction in health is justified given that it is the smallest emplacement and has the most health.
I reduced both the Mortar Pit's DPS and health by 25% to justify the 25% reduction in price to this forum. I agree that two 25% nerfs are a bit much.
Thank you for the correction on the Pak43's pop cost. I still think 15 pop is better for heavy AT guns like the 17 Pounder/Pak43 but that's an issue for a "Fixing the OKW" thread. What do you think of increasing the 17 Pounder's health from 900 to 1000? It's less problematic than a 1000 health Bofors due to its limited target selection and would let it take one more AT gun shot (two more under brace).
Please suggest some changes to my emplacement suggestions. I don't have as much experience with the 17 Pounder and Bofors as I'd like.
I love a serious discussion about emplacements
Bofors:a static anti air,Anti light vehicle(not light tanks),Anti infantry that is durable and can attack only units in its range and also needs side tech (which you want to make mandatory for next tier which is nice).
This unit can be killed with mortars (which both axis factions have)since the bofors can't attack them from such range and light tanks(not armored cars or vehicles) and last flame weapons (not alone) which decimate emplacements.
And that's units that arrive sooner and/or at the same time as the bofors.
So cost wise and counter wise the bofors is pretty balanced from that point only the stupid barrage needs removal as it breaks the cycle of balance and a size decrease so it can be placed in many more places.
17 pounder: static,heavy at gun,durable.
You covered it your self,this unit only attacks vehicles/tanks and is weak to infantry as well as mortars,artillery/rocket artillery.
10-12 cap would be helpful alot more than 15 cap(which is still insane for a static at gun) and then a size decrease to help place it cause currently it's a nightmare to place one while also taking more shoots from artillery which in turn increase the damage it takes,after that its either a slight price decrease or a durability increase or allow it to have a Vickers inside.
And lastly mortar pit
It's a more balanced sniper that is more dreadful to face,this unit doesn't like moving targets much and also has minimum range,it would make more sence to make it cost 260 and be an emplacement with one mortar with the ability to pay extra for another one(current double mortar pit) and allow it to be built early with out killing gameplay and be build from sections and upgrade with cost after you tech,it's also a nice idea to split durability with the extra mortar upgrade (half current durability with one mortar,current durability with second mortar upgrade).
Durability nerfs are just bad cause it basically kills their strength (sacrifice mobility for durability) and also doesn't take into account the extra damage they receive from certain sources while also their inability to react to anything (since brace won't really help the emplacement after that).
And think of brace as a retreat action to allow emplacements to survive oneshot abilitys (railway,stutm tiger shot).