Just edit and put a space after the 8. Then instead of (18) you get (18 ).
Burst of Strength ought to require that the user also be wielding a weapon 2-handed. This way, someone using a hone handed sword normally wouldn't be capable of it - he'd have to grab it two handed. I'll warn you though, messing with STR bonus can be a little too deadly. I'd suggest the following:
New Feat: Mighty Swing
The weilder of a two handed weapon (or someone using a one-handed weapon with two hands) may make a mighty swing with all thier might to hew opponents that normally might be more difficult to damage or in an effort to finish off thier foe in a final, single blow.
Prerequisites: STR13+, Power Attack, Cleave, wielding a weapon with both hands
Benefit: As a full round action, the Strength of the character is increased by his CON bonus for a single attack, after which he must make a FORT save at a DC equal to the damage he deals or be considered fatigued (-2 penalty to STR and DEX and cannot charge or run) until he rests (takes no actions other than to stand or sit, not even using Dodge or Parry Defense) for one round. A character may use Mighty Swing a number of times per day equal to his STR bonus but only once per combat.
I just thought of this one too:
New Feat: Hewing Strike
The efficiency of the warrior's strikes has been honed to an even deadlier precision, lopping off the limbs of enemies on nearly every strike.
Prerequisites: STR13+, Power Attack, Cleave, Great Cleave
Benefit: The character can elect to make a hewing strike with any one of his attacks in a round prior to rolling to hit. For that strike, the weapon's threat range is increased by 1. If the range was 20, then it increases to 19-20. If it was 19-20, it increases to 18-20, and so on.
Special: This feat may be taken once for every five character levels. For example, a 5th to 9th level character could have it once, while a 10th to 14th level character could apply it twice, further increasing the threat range of the strike.