We had several engines at the junkyard we un-seized,that were running when the vehicles they were in were hauled in,but sat too long--some for up to 20 years..
--most of the ones that refused to break free after soaking them with ATF and diesel in the cylinders,by using a breaker bar on the balancer bolt,we were able to get to move and break free, by using a long pry bar on the flywheel teeth..we had to lift it up with the loader to do that..
Once they broke free,we were able to turn most of them back and forth,a little further each time,until two complete revolutions could be made--many of them actually spun pretty freely once the initial "bond" was broken....a few vehicles that had manual transmissions we towed and dumped the clutch in second gear..
Some of the engines fired up and ran good,others had stuck valves,and they ended up getting smacked by the piston..others seemed to run OK,but only lasted a short time before they started belching smoke,probably rings got stuck,or possibly broke during the un-sticking process--a few seized up again and a few threw a rod..our fault for not putting fresh oil in them probably

.....one 289 Ford had the camshaft snap after it ran sweet for about 20 minutes..
Out of about 20 engines,I'd say at least a dozen survived,and ended up being good runners..the rest either needed rings,a bore job,or were junk due to cracks,from water that got in them and froze..the ones in vehicles that still had coolant in the radiator were the ones that fared the best..one of them was a 425 Buick nailhead from a '66 Electra,I wanted that one bad,but it got claimed by an employee when the yard closed up for good..
One of my friends had a '72 Nova given to him long ago,that had overheated badly and seized up...it had a 250 straight six in it...it was bound up good,we could not get it to budge using a breaker bar..--he wanted to see if we could get it to run long enough to get it to his house 4 miles away,rather than pay for a tow..we pulled the spark plugs out and dumped ATF and marvel mystery oil in the cylinders..and we let the car sit a few days..
He had an idea,he took a 2 ton hydraulic bottle jack and put it under the flywheel teeth after removing the tin shield,and he jacked up against the flywheel--the car started lifting up,then it broke free!--we were able to get it to spin over with the starter with the spark plugs out and some more marvel mystery oil in the cylinders,and we spun it over a good 30 seconds to make sure it had no stuck valves..
It fired right up after we put the spark plugs back in it--I followed him to his house and got smoked out so bad I almost threw up,but by the time he got home the smoking had almost stopped..
The car needed a radiator,and he got a used one and put it in,and drove the car short distances for awhile to see if the head gasket got blown--it seemed ok,so he started trusting it further...he ended up driving it for 3 years before he came across a good running 350 and put that in it--the 250 was still running great..
I wouldn't just give up on the Buick engine yet..let it soak longer,and keep trying to break it free..it may surprise you..also make sure the water pump,alternator and any other thing like an air pump isn't seized as well--we had a few engines that had seized water pumps that spun right over after we took the belts off!..we almost scrapped them...
I've read about some guys who used a grease gun and a adapter made from an old spark plug with a zerk fitting in it to pump the cylinder full of grease,on old tractors and one lung air cooled engines,they claim the 10,000 psi pressure works when all else fails..the bummer is getting all that grease OUT again,without having to pull the head off..