If your truck stays in Seattle, you probably won't need a heat riser. The riser in my 235 is stuck open, and I've had no problems getting it started this winter. The temperature here is similar to Seattle - ooccasional frost but rarely below 20F.
As for your gas mileage, 15 mpg is about average for these old engines. You might get a few more or less mpgs depending on how well the engine is tuned.
