Put a ls engine into it.
Everything bolts up and it is easy to do. Can be done for around $3000 correctly starting with a $500 150k engine then replacing seals changing mounts, exhaust etc.
There are many custom things that you can do with a square body. Engine wise a ls just works. I have a 02 which is a "good" year swap wise for the 87.
300hp and truck torque with lots of potential and around 14 mileage. Easy wiring hookup and very reliable.
Budget wise find someone that has a "built" 350(355/383) and is selling it for around $1000 air cleaner to pan. They are out there I have bought over a dozen "built" engines for around what the heads(dart or other) cost alone. Run the engine, do a compression test, then tell the person to pull the cap and leave the dizzy in when they pull the engine. Use a lift plate on the intake and your good. Leaving the dizzy in saves some time and you don't need to wonder if they threw the thing across the shop floor. Thing will be a weekend swap and easy to do. Most of your stuff like exhaust,, fuel, cooling, mounts will all bolt up so no messing around fabricating. Even with new seals you should be able to buy and drop in the newer engine under $1500
Around 250-330hp and requires some tuning. But mileage wise your still over 12.
You can go big block. But really there are a few issues here.
With a 472/500 your running into the valves, timing chain, and clearance issues of the pan and oil filter. These engines are pigs so don't expect mileage over 8. Furthermore the "good" high compression engines require premium and also require a re curved distributior to go from points to hei. They make great torque but only till about 4000 where things start to get funky. Fixing these issues is not cheap at all because it is a Cadilliac(same goes for the buick and olds big blocks of the 70's). You can easily have 3k into a 472/500 swap to make it reliable on todays junk gas.
With a 454 you have a wealth of parts and potential. Not to many headaches dropping it in and they are easily sourced if you ask around. It is one of those engines you can build upon or run around with just 300hp and a bit more torque then a 383. Mileage wise 8-10 depending on your foot. But back to the parts part of it a 468 makes good torque while having a nice amount of reliability at a fair enough cost. Finding a "built 454 is a bit tougher but if your willing to play with peanuts where someone swapped in a rv cam you can throw a carb intake onto that with hei and then be good to go for under $2000 after you do the seals and such.
You can go the other way and go smaller. I have had 4 full size trucks sporting 4.3 engines and one sporting a 3800 engine. They are good, not great, but good engines. But you need gearing to make them work. Most of these came with a manual 5 speed in the full size trucks and gearing around 4.11. The thing wont be powerful but it is easy to work on and very reliable. Parts wise though your screwed in most cases. Which drives up prices and Mileage wise your going to be about the same as a 350 due to having to work that engine harder at slower speeds if still using a automatic.
Screw ICE and go electric. Now you need to do some homework here. But Building your own parts, using common packs like toyota, sourcing junkyard motors, and just talking with people who have done it and learned from the mistakes you can do a electric conversion for Under $5000 with a range of around 60 miles(2 packs). The range sucks here with a 220 recharge time of around a hour for every 10 miles. But the mechanics are stupid simple and if your just driving the thing around short trips it may be a option. Heck throw a generator in the back and you have a hybrid when parked.
Battery packs are quickly getting better year by year which will allow your range to be extended closer to that 100 mile mark within 6 years.
If you want to run a hemi I would just run a 727 and a 205 behind it. Price wise nothing about a hemi is cheap. Your going to be over $5000. I would budget closer to $7000 to do the swap right pushing around 500hp and the short version here is that a $7000 LS motor running some boost will make similar if not more power while being cheaper to run and repair.
Asking around and finding a "built" 350 will keep things simple and make this a weekend tbi 350 to carb 350 swap. Ease of use wise that will get the wheels moving on this for the least amount of fabrication and cost. It will allow you to keep a eye out for a great deal on a power train to do a swap with later.