Cylinder Head Choice

I’m slightly confused about the gear ratios and the torque converter. Would I need to buy a new rear end before getting the converter? Is that the only way to change gear ratio to 4.11?
No, but you would get a new converter to be used in conjunction with a new gear ratio and the camshaft to be used.
This converter seems to fit all of the specifications you suggested as well - would this be a good one? thank you for all the help!

TCI 751400: 10" Streetfighter Torque Converter 1972-80 AMC Torque Command 727 | JEGS

No, step up and pay the piper and get a great converter rather than a generic band aid.

Also, IMO, save money and don’t get the aluminum heads. Your not so doing much with the engine that the iron head prepped can’t handle. If you’d going to step up to the plate and build a serious engine for a heavy street hitter / drag car as you stated earlier, your not getting there from here with the 318 as it sits now.

Your also pretty unclear of your path or plan.

Let’s start with.... how fast is it you want to go in the 1/4 mile?
What kind of budget do you have?