Swap gears or swap engines?
 

This is slightly eco related, since my goal is to have lower RPM on the highway. So I have a bone stock Civic VX that is a blast to drive, and gets great gas mileage. I love the way it looks and have wanted the same body style but with modern sports car power and speed. I’m picturing a grand touring style build with all the creature comforts such as air conditioning, leather interior, comfy ride and good sound system with sound insulation too, and it will see many 1000+ mile trips and frequent spirited driving in the mountains or canyons.

For that purpose, I bought a second VX with a B18C5 (Integra Type R) swapped into it already (got it pretty cheap, I originally only wanted the shell). My original plan was to build a K24 with turbo and get around 350-400 whp, but gear it tall enough to keep similar highway RPM to my VX, which is about 2600 RPM at 80 mph. Literally every motorsports shop I have talked to has told me to keep the B18 and swap in the 5th gear and final drive from an Integra LS, which will help improve it but will only end up with 3400 RPM at 80 mph, higher than even 4th gear in the VX. Can I find a gear set to work with the B18 engine? If not, will I have any better luck with a K24? I don’t want to stay work on this car until I know which engine and transmission I will end up with and know it will meet my end goal.

I don't think the (earlier) K24 is a big improvement in efficiency over the B18, and the much higher displacement is not going to help gas mileage.

If you want to keep the body style then I would just do the engine swap. It will definitely be more work and money, but you will probably be happier at the end of the project. Not all choices need to be the most logical one.

On this site, user Ecky built a k-swapped first generation Insight. He built it for both the power enhancement and the eco potential. Think of it as a heavily aero modded Acura made to look like an Insight for the drag benefits. But he also swapped the gear box for taller gearing. Hopefully he will notice your question, but if you take the time to read through his thread and contact him you'll likely benefit. I think he swapped in a 3.4 FD from a compatible gearbox and got a major freeway ROM benefit while still getting the power he wanted. Most radically eco-modded Acura we have ever seen on this site! :)