I have the same Volvo 280T drive that you have. I had an original Hill Bronson Thrasher, but it was a 24". I liked it up to about 45 mph, but after that it seemed to give too much stern lift and felt squirrely. I subsequently sold it. I have a Solas 21 and a couple Ultra's both a 24 and a 26 that I found new in the box, which work much better on the 18 hull. I actually prefer the Ultra's, as they seem to provide more bow lift but the Solas works nicely too.
1978 X-18, SBC, Volvo 280T
Big Grizzly 21 Solas Prop
Heart Bay, Lake George, NY