Mine is a 2003, but should be very similar to your '98.
I posted pictures of the process in getting to the rear valve cover.
I replaced the valve cover gasket and the spark plugs.
I posted pictures that show the cowl removal, Upper intake manifold removal, to get to the rear spark plugs.
Here is a link to the first in the series of pictures of the upper intake manifold.....starting with draining some cooant from the system.
You will need 14mm, 12mm and 10mm sockets and a 8mm hex bit.
I replaced my thermostat the same day.
I do not recommend doing both projects in the same day.
My thermostat was regulating at 163 to 168 degrees.......instead of the 180 degrees that it should have been at.
TOYOTA states that the 180 degree thermostat should not start to open until at least 176 degrees.
YES, the one that I took out was marked "82" for 82 degrees C.
I bought the replacement thermostat from TOYOTA, as well as the 2 "O" ring gaskets.
I ran the Michelin Harmony tires on my '96 windstar (sold that today...super good condition, but over 220K miles on it) for years and was very happy with them.
My Sienna has the larger tires, and I had my parents try the Harmony tire on it when they owned it.....and they were very happy with them.
It is what I plan to keep using on it......unless I get a itching to try the Hydroedge.
My parents, who live over 500miles from me, kept worrying about me driving around such an old car.....so when they decided to replace their 2003 Sienna Symphony with a 2010 Sienna LE, I bought their "old" car.
I know that a minivan is not "cool" nowadays.......but it suits my needs.....and I REALLY like the Sienna.
I am CHEAP in that I hang on to a vehicle for a long time......I make it a hobby to keep it in good shape.
I am not cheap as far as my preventative maintenance....I try to do routine things myself, using the best quality materials that I can.
I drove my windstar for 11 years.......
A friend of mine told me that if I got so many years and miles out of a windstar, the Sienna should last me the rest of my life.