I own an '86 M1008 with the 6.2L diesel (35K miles), and an '85 M1028/C-10 project truck that'll be getting a 305.
The 6.2 is a dog, but I don't mind that. For me, the truck is primarily used as an around-town truck (30-50 mph speed limits) and somewhat of a beater farm truck. I'm consistently getting 16.5-17 mpg, which is pretty respectable for a 1.25-ton truck, IMHO.
That said, when I first got the truck 7 months ago, I checked with several local diesel shops to see if they'd just do a basic "health check" on my new truck to look for any glaring maintenance issues/considerations, and had shop after shop tell me they won't even work on them, as parts availability and prices are tough. I get that, but I was just looking for a basic mechanical inspection. Still, no go. I finally did find one shop to look at it, but that was 6-7 shops after I started looking, most of which were diesel-specific shops.
My 6.2L now has a Fluidampr and some new pulleys (which has gotten rid of my previous belt tension issues), new belts, new CDR valve & oil cap, etc. and it's no longer a "ticking time bomb" with the OEM harmonic balancer.
So I do like the 6.2L, but don't know if I'd buy another (at least non-military) truck with that engine. If I was still making my own biodiesel fuel, which I did back several years ago in the Obama/$4+ per gallon fuel days, I'd obviously be biased towards the 6.2L for that reason alone.
The 305 may be a bit of a dog, too, but it's at least better supported than the 6.2L.
That's my $0.02...worth what you paid for it.
Craig