This conversion sounds astronomically expensive to get approved.

Take the money you save by not doing the conversion and use it to overhaul the existing turbocharged engines once or twice and still fly the thing down low for the rest of your life... Coming out ahead in the bank.

I should have added that the 310 version only has one high pressure electric fuel pump per side and as you go higher (with them off) the fuel flows start to fluctuate giving the engines a surging with each fluctuation.
I crawled under a 350 model (because they have a low pressure pump AND a high pressure pump system) to see how it was mounted. Surprisingly the entire wing root system would fit in the 310 with just a couple of factory mounts riveted on and wires and switches installed. I got the parts and installed a Chieftain system in the 310 with a field approval only and from then on whenever I went high I could use the low pressure constant flow pumps to even out the fuel pressure to the injector. Worked like a champ.
A great airplane, a great engine package and one that won't (if maintained properly) nickle and dime you to death.

I've owned a 310 Navajo for 10 yrs now and absolutely love it. I am retired AF fighter pilot, retired airline, and currently fly corporate jets so I'm pretty serious about planes. I generally fly it at 60% power to increase longevity of my engines as well as saving fuel. The fuel distribution on these engines is so good that I can run lean of peak without GAMIS. Typically I fly between 5 and 10 thousand feet depending on winds though I have occasionally gone as high as 15. I don't like to wear canullas or masks so that's why I stay this low. Today on takeoff, tower warned me about many flocks of large birds near the departure end and suggested a steep climb. With 5 adults on board and 148 Gal of fuel I was climbing at 1800 fpm through 2000 feet. At 8000 feet I cruised at 175 KTAS on a total burn of just 26 GPH. We normally leave the outboard tanks half full to increase payload by 240 lbs. With that fuel load we can haul 1264 lbs of people and luggage 740NM with a 1 hrs reserve. With full tanks we can go 1050 NM with a 1hr reserve, but can only carry 1024lbs of payload. It was my wife's idea to get this specific airplane because as she said we could carry the kids and grandkids. We normally fly it with the 6 regular seats plus the potty seat, though we do keep the 8th seat in the hangar in case we ever need it. If you need to carry a lot of folks a long way safely this is a great machine for it.

KW - I have read through this entire intriguing thread. The proposition intrigued me because it is something I would never even consider, and therefore wanted to understand the reasoning.

My 2 cents would be to echo what many have already said in previous posts. If you can find the airplane you want (i.e. NA engines), buy it. I would not go through the irrecoverable cost and hassle of "downgrading" one of these. The expense and headaches you will go through converting an existing airplane will far outweigh any cost savings you are seeing. These are old airplanes and the moment you start opening stuff up, it is almost guaranteed you will face new "discoveries", and by the time you are done, you will wish you had never started - especially on a mod that diminishes value rather than enhances it.

Also this:

I want a color navigation, terrain, traffic, as well as weather displays. Full weather capability, good autopilot with a Flight Director, and an altitude/VS preselect.

will probably be either completely or partially absent in any NA Navajo you find, as it will probably be a neglected old bird which has been flown on a shoe string budget. The guys who can afford these niceties, usually were flying the more capable Turbo Navajos. That means you will have to incur additional cost in upgrading the airplane which is additional sunk cost.

I would suggest rather finding a very nice Turbo Navajo with solid engines and good avionics and start enjoying your traveling machine. A properly flown Turbo Navajo in experienced hands like yours and treated with care and good judgement (you can always throttle back you know), will last a life time if the engines are solid to begin with. These are rugged birds - I don't know that I share your concern with MX costs if the engines are treated with care. Furthermore, the turbos will always provide options if you need them. Plus, you'll be able to quickly flip it the day you decide to move on.

Sometimes its hard to change our minds once we are set on an idea, but perhaps the cost savings you are seeing are a mirage once you take the all-in cost into account and the headache may be significantly higher. Just my 2 cents.

who knows, maybe someday I change my mind altogether.
KW

Hey Krzysztof,

Have you seen the Twin Bo forum on here? Just sayin'...

We normally fly it with the 6 regular seats plus the potty seat, though we do keep the 8th seat in the hangar in case we ever need it. If you need to carry a lot of folks a long way safely this is a great machine for it.