Another vote for the 20'.
The bare hull weight difference is only a couple of hundred lbs so it shouldn't be a big difference for the little Toyota.
That being said you will be at the upper limit of safely towing with that truck.
The weight capacity of the 20 is about 320 lbs more than that of the 18. That's a lot of beer, gas or gear, or a dude and his petite gal
I like a closed transom (and I'll probably hear it for this) but due to its narrow transom beam at waterline, I don't think the Seacraft 20 is the best candidate for a bracket. 25" transom and same shaft would be my choice.