A few years back I owned several Class C's with the Dodge 360 and Torqueflite 727 tranny, they were almost bulletproof and gave me great service. I kinda miss them old girls. I also had the Winnebago Class A's with 413 Dodge Truck Engine NOW THAT WAS A GREAT RV ENGINE. I bought an Edelbrock replacement carb for one of those but its been too long now to remember just what one. However the Holley 500 you mentioned with elec choke sounds like the easier way to go.
To improve MPG on an old carburetored engine, Holly or someone used to make a sort of Throttle Body Fuel Injection replacement device that looks like a carb or later model throttle body injection system that bolted right in and replaced carb. HOWEVER they were like $700 so not sure if a good investment. I owned a ton of those old Dodges and the bet way I improved MPG was to install true dual exhaust with each side having 2 1/2 inch pipes and each side like a Thrush or high flow muffler and tail pipes I shortened and ran out the sides versus all the way back. A bit louder but did improve MPG.
Years ago I sent my carbs and distributor to Gerharty Automotive RV in California and they adjusted the weights n springs in the distributors to change the advance curve plus laid hands on the carb and did some magic and that along with my free flow dual 2 1/2 exhaust pipes added at least 1 MPG. I would also use 4 inch aluminum dryer vent hose with a brass fine mesh screen cover (keep bugs out) and place it so while driving it was forcing air into the air cleaner assembly. I just placed it somewhere for direct incoming air while driving and hose clamped it over the air breather intake. Sort of like a poor mans blower or supercharger.
What I did they prob would have thrown me in jail if I drove in California, but let the engine breathe n take in more cool air and less backflow exhaust restrictions and they gotta get better MPG . Still in the modern world now with my 454 Chevy direct injected Vortec and 4L80E Overdrive Tranny I can get 10 MPG in my 29 ft class C and that's better then the old days of 8+
A holley 500 cfm w/ electric choke is about the best way to go. Its a simpler install, and the metering jets are easier to adjust. Being in a higher elevation you need fuel flow to match the lack of air flow. You mentioned a 600 cfm,, That in my opinion would be a little to much ummmph in the fuel to air ratio your going to experience.
John T. did make an excellent point using DUAL 2 1/2 inch exhaust. I put dual exhaust on my one ton chevy dually but I went with 3" pipe. It opened up the engines ability to breathe a bunch. Could actually feel the engine pull and excelleration was better improved. As for intake breathing,, Doghouses restrict air flow. I have used the dryer duct trick, and many people laughed, BUT it did work. The engine didnt have to rely on what air it could get by what ever wind happened to come into the engine compartment. The duct hose is like a poor mans mini turbo. You will feel a noticeable difference when you first drive it. It will have a little more ummmph and will actually run better.
Edlebrocks are OK carbs,, But are limited in my opinion as to rejetting or even getting the correct jets other then having to go thru the company,, and by then, you lose warrantee when you rejet from a factory setting. I have had good success with Holly and there *Hotline for help.
The Dynomax exhaust is a decent designed muffler,, falls into the old school hush thrust mufflers we used back in the day,, (They are like the Banks version of a turbo muffler with a flow thru design), which is to tone down the exhaust noise but giving back pressure to avoid blowing out exhaust valves. (Old school was easier to work on when we had regular fuel available, at least the lead cushioned the valve seats).
I'm leaning more towards the 2 barrel Holley. What is the general consenois?
Dynomax sure decent old school. What is a great muffler present day? Need help getting up to speed. Been way to long since I tinkered with this kind of stuff.
New formulated gas has been a challange. That's another subject.
Thrush makes a decent old school concept muffler, We call them hush thrust,, sounds throaty when you throttle into it, but it tones down the rattle and rumble when you let off.
Forgot to mention, A Thrush is whats on my Diesel, and its running off a 4 inch exhaust pipe into a 6 inch stack. its actually quiet.
Ya, the Thrush is alittle throaty for my taste. Not bad though. There's new technology out there like the "Cherry Bombs Vortec" the best of all 4 muffler designs. A little pricey but if I'm able to bump up mileage even if its 1 mpg. could pay for itself with these fuel prices now adays. Just alittle concerned at the db (noise) level.
True on the noise,,, Its not good when they can hear you 8 blocks away. Plus in an RV park, there are day sleepers that get annoyed when ya rattle there windows just idleing thru,
Let us all know what works and post it.. Im always up for better.