Structral strength boost does not increase your health or makes your ship stronger. It just enables your ship to still overheat/not lose speed when heavily damaged.

You'd better try ammo damage +3%, turning force +1 and gun accuracy +1 (KM gun accuracy really sucks, though gun accuracy +1 does not make too much difference).

A addition note:
ammo damage +3% sounds nice but isn't really noticeable. Furthermore i believe it just boost the HE part of the damage , which is jsut 1/4 or 1/6 of totall damage that a HE shell does. So totally it isn't more then 1% more damage.
More usefull are +1acc (especially if your gunners aren't 120 yet).
The +1turning is a good addition for dodging and making slinging a bit easier. 

Well you won't be able to use 4 damage boosts because you can only have 1 of each particular tuning item in a ship at one time. So BBs can have 3 different parts, not multiples of the same ones.

Also, there is no component that increases ship DP as in health. There is one that increases ship defense power (called structural strength), but that is a measure of how much damage the ship can take before you lose OH ability and speed. The same function as bulkhead armor.

Once you slot an item, they last for 72 hours, or 3 days.

I'd probably go with a combination of Ammo Damage, Reload Speed, Gun accuracy and Turning.

Aside from not being able to use multiple tunings of the same ability on 1 ship, BB's can only use 3 tunings at a time.

ammo damage +3%, turning force +1 and R mount reload time -3%

R Mound reload time...only gives a 0.5 sec decrease of reload time for H44 guns?