The $30 to $40 grand seems high to me. Granted I replaced my 6-0 with a 'NEW' engine (not a remanufactured) back in 2015 for $32k. I chose a dealership since the warranty would be good across the country, private builds normally have to be brought back to them - which would prove a huge obstacle when you're 500, 1000, 2000 miles away!
The dealership I had it done at (just outside of Napa, Ca) had several Bullet Proof Diesel staff working with them & training their mechanics at the time, So, I took advantage of the opportunity and had the new engine completely bullet proofed, along with replacing everything in the engine bay (all hoses as well). The Dealership engine replacement involved reattaching all the others stuff as well, so when we bought all new components they didn't charge me for installation of almost 80% of the parts since the engine was out.
When you finally make your decision, I would inquire to see if you're able to get the same commitment - I saved quite a bit of money!
Member ShuttlePilot has a comprehensive list outlining a thorough Bullet Proofing which I suggest you review:
https://www.sportsmobileforum.com/forums/f33/6-0l-engine-bulletproofing-25269.html
The extras I had done (all included in the $32k price):
Engine was outfitted with all the Bullet proofing suggestions including the head studs, Aluminum radiator, tension pully, degas bottle, new steering box, new Turbo & all the pipes, OEM Trans cooler, Intercooler, water pump & thermostat, vacuum canister, all belts & hoses, a slew of sensors, fuel pump & regulator, go-plug harness, injector harness, trans harness and FICM, EGR cooler & valve, and more!- any of these available in BPD brand was purchased from them & installed on the new build.
The mistakes I made:
I didn't replace the Alternator (High output and they said it looked recent) - went out on me about 6 days after I picked up the van!
Blew the seams out of the first 2 aluminum radiators, and finally went with a Mishimoto Aluminum radiator...
highly suggest you also install the Mishimoto low temp thermostat (it allows the engine to run much cooler than the OEM - a real benefit in controlling high temps for these heavy vans)
I didn't replace the Engine wire harness - they said it was in really good shape, but despite that I still ended out with problems several years later.