340 gas mileage

Bassically agree ^^^^with Rob rumblefish
If you can install an engine and hook it up, you can change the rods in a carterbrock.

I just looked at the 1406 chart.
It starts with the largest diameter rod, so you can't go larger on the rod.
To go leaner, you have to change the rod and jet.
The chart is on page 12. The next leaner rodjet combo is listed on line 8.
https://www.edelbrock.com/pub/media/wysiwyg/documents/carb-owners-manual.pdf