You have 32.5% equity vs a range of K8s, K5s, 88, 55, AKs, AKo. Is this player really flatting $75 pre with K8 or K5? If he is, I think we have to also bake in some out-of-line equity. Also, can't he then have K7s? K6s? KQ? QJ/QT/JT/65 of spades? AA?
I think you are underestimating your opp's fold chances when you check-call turn then jam when draw hits. 2 of your 15 outs put both the flush and a 4-straight on board. What are you check-calling $500 turn and now jamming for 1/7 pot on a 3-flush, 4-straight board that AK beats?
But let's work from your assumptions.
If we fold, our stack is $775, $0 EV.
If we jam, we make a pot of $2075 which we win 32.5% of so our stack is on average $674, -$101 EV.
If we call, we will hit a spade or straight card 15/46 of the time or 32.6% of the time. If we jam the other 32.6% of the time (I def think we have to jam the 5s and 6s, otherwise we just let him check back when he has less than a boat and stack us when he has a boat), we win always 13/15 of the time (86.7%) and win 67% of time on 5s river and 60% of time on 6s river.
So 67.4% of the time our stack is $275 as we check fold our misses. 28.2% of the time our stack is $2075 as we jam when we hit other than 5s or 6s and get called and win. 1.6% of the time our stack is $2075 as we jam on 5s and get called and win. 0.8% of the time our stack is $0 as we jam on 5s and get called and lose. 1.3% of the time our stack is $2075 as we jam on 6s and get called and win. 0.7% of the time our stack is $0 as we jam on 6s and get called and lose. That adds up to:
31.1% of time $2075 and 1.5% $0. (.675 x 275) + (.311 x 2075) + (.015 x 0) = 645.33 + 185.63 = $830.96, +$56 EV. Note that if opp folds AK on river that's going to be 40% of the time he folds when we ht, which would reduce our EV roughly to +$30. So it's pretty close, and I'd say better than my initial impression, and profitable if your assumptions are correct.