I've got an 05 Toyota Corolla for most of the driving I do around town and other trips, it's reasonable on gas and certainly better than the truck.
The truck is mostly a project, but getting better fuel economy means my vacations cost less, and I get to my destination sooner. I do long-distance road trips mostly and car camp on my way to the destination, it's almost perfect for that application the only exception being the fuel economy.
As for grill blocks here's an example:
View attachment 110014
It's mostly done to reduce drag, some folks also do it to raise engine temps. Folks with diesels or heavy duty trucks do it in the winter as well to keep their engines warm in cold conditions which helps with fuel economy and performance.
Most modern vehicles are overcooled, especially on the highway, and that's why you can get away with blocking so much of the grill. You still have to be mindful of your temps but in general it won't cause any issues.
As for undertrays
View attachment 110015
This has been a thing on racecars forever, but effectively it smooths airflow underneath the vehicle improving handling and reducing drag.
The underside of cars, trucks in particular are awful for aero, so even modern efficient cars and EVs in particular will have fully smooth underbodies:
View attachment 110016
I'm planning on doing some aero work like the above to the truck eventually since it's part of the puzzle, but I need the time and $$ first.