I'm going to have to counter some of char's information. First off, a 500HP diesel isn't going to stress a 14b. 14bolts have a saddle bearing on the pinion like a 9" Ford, and this makes the R&P much stronger than an equal size overhung pinion unit like the D70. In fact, the "weak" link on a 14b, which isn't weak by any stretch, are the axle shafts, and those same shafts are used in the 11-½" AAM axle. The 11-½" unit, which is used on 3/4 and 1 ton GM and Dodge diesel pickups, does NOT have a saddle bearing like the 14b. It is a conventional overhung pinion design, and neither the pinion nor the pinion shaft is any larger than the pinion and pinion shaft on the 14b. I could go into a dissertation on why larger R&P is needed for an axle rated at higher GCWR, which has to do with how much tooth contact area the load is applied to on a continuous basis to keep localized temps down for durability, but suffice it to say you won't break the R&P on a 14b and if you break a 14b shaft then you would break an identical 11-½" AAM shaft. As long as you don't plan on towing over 18k GCW or so, the 14b will be perfectly fine. Note that 18k is very conservative, as second gen Dodge guys have routinely towed mid-high 20k range with the weaker D70 without any R&P issues. While I don't mind putting more torque through an axle than it is rated for, I like to stay within the GCWR limits to get essentially indefinite life out of the axle. Also, the 14b and 11-½" AAM use the same pinion yoke, and the 11-½" axles came with 1480 u-joints, so a 1480 yoke can be installed on the 14b if you find you're eating up 1410s.
Forget a 3000 series Allison. These are just too large for a pickup, and the smallest flywheel housing they accommodate is SAE #2 which is also too large for most pickups, unless you don't mind breaking out the Sawzall on the firewall. In stock form the 1000, 2000, and 2400 series have pretty modest ratings, but GM has increased the ratings on the 1000 series to 445HP, 910lb-ft of input torque, and a GCWR of 31,300lbs. Contrary to popular belief, while the 2000 and 2400 series have higher ratings, they all are identical internally except for gear ratios and presence or absence of a park pawl in some models. The park pawl has a weight limit, so it reduces the GCWR of the trans relative to the units without the park pawl. The transmissions with the deeper 3.54 gear sets reduce upstream load, so those transmissions tend to carry higher GCWRs vs the transmissions with 3.10 gear sets. Other factors are determined by TCM programming, which is in turn based on vocational use. Any Allison can be built to handle 500HP/1000lb-ft or so, but its really best to have working torque management. Allison 1000/2000/2400s typically use SAE #3 flywheel housings in non-GM pickup applications. The GM pickup 1000's use the standard GM corporate (aka SBC/BBC) flywheel housing. There were also SAE #2 housings available for these series, but they seemed to be rarely used since the converters on these smaller Allisons fit in the smaller #3 housings. If you are DIY rebuilding an Allison and no hard parts are needed, then the cost is comparable to a 4L80E. The biggest single expense is a good triple disk torque converter. Ally's usually don't need hard parts because they are robust internally. If they do, the price really depends on whether you can locate good used parts or have to spend the money for new. FWIW, Allison wholesaler outlets are MUCH cheaper than GM dealerships when it comes to hard parts. Clutches and steels are usually obtained in aftermarket kits that come with other mods.
500 diesel HP in a 4L80E (4L85E really) is problematic because such an engine will make around 1000lb-ft or more. There are shafts available that can handle that much power, but the main limitation is the OD roller clutch. Especially the later transmissions that uses the clutches with smaller diameter rollers. The torque will eventually blow that roller clutch apart. It's also difficult to get the OD clutches to hold that much torque. It's doable, but like I said even if you get that done it will just blow the OD roller clutch apart anyway. The most tq the 4L85E handled in stock form was 520lb-ft-lbs. That was when the trans was used behind a detuned Duramax in the Express vans. The only reason it was used was because the Allison would not fit on that van chassis.
It's much easier to build a 47RH/47RE/48RE to hold the power because they came behind the Cummins as the stock trans, and the aftermarket has made huge strides to get these transmissions to survive at high power levels. If you're fine with a 4 speed automatic. I'd stick with one of those, built for your power level.