The problem here is you can't be that sure he has the straight. you say 75%...i think the reality is much less likely than that. for a start the sb and bb both being dealt 78 is pretty unlikely, and there must be alot of hands he would play the same way. even if he had the straight in this situation, alot of times he won't

his raise on the turn looks like he's protecting something - it's a big raise. this could be with a set to make you pay for drawing. in the blind he could have flopped a junky 2 pair like 10-6 (and even with pt you can't be sure how he'd play it), or he might have turned some kind of mega-draw/pair combination.

your flat call on the turn that might seem oh-so sophisticated might in fact (more than half the time i'm guessing) be letting him draw cheaply for filling up, or making his flush/straight.

it would be different if you had a str8 with one of your cards, say u had k-8 in sb board was 9-7-6-10, and you are faced with a strong reraise on the turn from a tight player..it's pretty likely he's got the straight, so flat call is ok here, and push the river if the board pairs.

but in future - you can't be so specific in your reads, alot of people mix up their play and might play a flush draw as they would top set as they would ace high on a bluff