Easiest/Cheapest way to a 4bbl 318?
I would get the engine done first and and bolt the headers on.
Start the engine and break the cam in and when you have it running good and you have no leaks,take it to the muffler shop.
Your car will run fine with the gears you have in it for now, but just try to do everything as cheap as possible first with the engine and exhaust.
Don't order anything ,other then the stuff you need to get all this done first.
THEN.........see how much money you have left over.
ALSO.............
You have to buy a new timing chain set.
Fuel line for the new four barrel.
A four barrel air cleaner.
Plus all the gaskets.........intake\header\carb\cylinder head\valve cover\timing chain cover gaskets and crank seal.
antifreeze\oil and filter\plugs
You can find headers for cheaper then $400.
Some shop should be able to make up a exhaust system for about 150.
This a system with no tail pipes and $29. mufflers.
Should cast no more then $220.00
Randy's ring and pinion has pretty good pricing.
Ring and pinion $155.00 per set.
Posi...anywhere from $380 to over $600.
It depends how many splines your axles have.
You will most likely have to pay a shop to install the new gears.