I'd go with the Holley 600,demon carbs are "garbage" plain and simple..thermoquads or "thermobogs" as we were refered to them back in the day aren't much better,then again i wouldn't trust a "plastic" carb....
Well who wants to run a carb like that,simplicity is always best..we use to take the "thermobogs" off and throw them away i can tune a Holley or better yet my Quickfuel very easily, thermobogs are a pain to tune and adjust certainly not worth the trouble..
While you have excellent points, I also will agree to disagree here on the T-Q's abilty.
I'm getting a little more appreciation for that 'complicated-plastic carb'. I may try a TQ. They seem to be pretty cheap for the reman'd ones & I also dig what you said abt the lower fuel temps - I do use E85 quite a bit, so that's appealing. Just gotta make sure the gaskets can handle the alky fuel.
Contact "Demonsizzler" here for a better carb than a "Re-man" garbage unit. His prices are very good and so is his service.
One question: Wouldn't a TQ be a little too much carb for a 318 (also, I've got one hella built 273 to consider as well)?
No, the small primary T-Q is an excellent choice for a stock/street strip small engine. The bigger primary T-Q may very well be to much for a mild small cid engine.
Maybe I'm opening a can-O-worms beyond the scope of a web discussion, but is there an economical way to get those ratios & diagnostics? Or maybe the better question is: How do you 'plug-in' to say, software to actually give readings? Is it as simple as tapping a sensor into the carb, or manifolds?
I imagine that to get into that kind of tuning, one has to get pretty serious about carb tuning (& maybe do it for a living).
I'll first start with the cheapest way to help tune your carb;
A simple Edelbrock 02 sensor. Weld a bung onto the headers collestor (Not reducer) and screw in the sensor, plug into the monitor and a simple 3 wire hook up and your reading the exhaust.
A more expensive unit but much better is this one which can monitor both sides of the exhaust at the same time. A little pricey, but worth it IMO;
http://www.summitracing.com/parts/FST-170402/
Unless your wanting to measure the exhaust directly in terms of what exactly is comeing out the tail pipe, a auto shop is probably the only way your going to find out since machines that do this are not exactly for sale at the local speed shop. And used one are a bit pricey for the home Hot Rodder like us. You could search E-bay or other places to find one.
About carbs
As I agreed with Stroked 340, a T-Q can very well be a pain to tune, even more so if your not used to dealing with them. Taking them apart is a pain to get to the jets though rods are super easy to change.
Every point of the T-Q has a measurement that must be met in order for it to work correctly. This carb is a little touchy in this manor. If something is outta wack, the carb acts ill.
One thing about a T-Q is it's enemy, HEAT! The plastic main body of the carb can warp/crack with to much heat. The best defense aginst heat are the basics. The T-Q came from the factory with a thick gasket. This should be used at all times. Blocking the intake manifold heat is another plus. While it hurts warm up times, the extra heat from the engine due to increased cam size (Heat from more power being created) and headers (Direct heat) can hurt the carb and should be delt with so it doesn't hurt the carb.
Of the few draw backs of the T-Q, the one that upsets me the most is the lack of electric choke or the pain in hooking up a manual choke cable.
Electric choke T-Q's are out there and there all the small primary version. Mostly seen with a "Chevy" arm, a MoPar armed unit is out there.
The Quadrajet, the other spreadbore carb, is also a very good carb for the mileage power search. How ever, no matter what carb you choose, proper jetting/timing, fuel octane will get you the best mileage out of the carb. After all, a poorly jetted carb of any style will suck for mileage and power.
I myself am used to dealing with the Carter carbs. Being part of the "Poor boy" racing club, you learn to deal with what you can afford. Once I had a Holley and broke the base plate. Since (At the time) I paid $200 for the carb, they (All the speed shops) wanted $100 or better for a base plate. Sorry, I just can't afford this type of price on a replacement part. I'll get the whole danm carb for $20 at a swap meet.
So I learned the OE carbs. And they perform preety good for a stock carb. After awhile, I just couldn't see spending money on a Holley when I'm standing toe to toe on the street with my $20 junk pig iron. rebuilt with a $25 kit.
That being said, In my opinion the best carbs for milage and power were the spread bores, ThermoQuad and Quadrajets, when properly set up. Another feature that a lot of people overlook is the adjustable air valve setup on Carter AVS and Thermoquad, as well as the Quadrajet. This allows one to slow or speed the secondary opening perfectly to the needs of most engines, Especially 273's and 318's. 
