maybe you have 140? thermostats in it? You have the correct gear ratios, and a 15 p prop is about as low as you want to go. You have other problems if it won't get on step, that or it's not a planing hull boat.
I bought the boat on eBay...it's the first I/O I've owned and you guys have been great helping me get my feet wet.i'm running a 160 deg. stat. in my aq260. I'd love to see that houseboat doing 30! Cool boat, btw. What is the condition of the motor? Compression numbers?
A 160 is correct. My bet is that is what is in there now.
BTW, did you have the prop looked at yet?