The 66 I bought for my daughter sat 10 years. The engine was seized solid. It would not budge not matter what, including putting a 5 foot pry bar on the crank and trying to turn it. Common issue with old engines that have sat, this works for push rod or over head. This will work for any car, truck, farm equipment etc...
24 hours later it was turning, and other 2 days and it was running.
Here is how.
DO NOT SMACK THE CRANK WITH A SLEDGE HAMMER, you'll just break things.
Put a bunch of scrap cardboard under your car as a bit of a mess will be created.
Mix up at least a gallon of the following (1/3rd each). Marvel Mystery oil, deisel fuel, automatic trans fluid. Pop the oil drain plug and drian everything (look for water in the oil so you know the condition of the head gasket), then put it back in.
Pull out the spark plugs (remember to note which high tension goes to what cylinder). Now with a funnel pour the mix into each cylinder. Every hour or 2 refill them as it will works it's way down into the engine. Also load up the top end (valve cover oil filler). Let it sit overnight. NOTE: valve cover gasket is probably fragged, so little bits.
Now put a big 'ol 1/2 socket monster onto that crank, slid a pipe over the handle so now the handle is about 4 or 5 feet long (use a strong pipe!). Put your shoulder into it and slowly try to turn the engine. Stuck still beyond movement? Then repeat soaking the cylinders, you may need another 24 hours of that.
Once it is turning it will do so very slowly, and be tough as hell. Just keep turning, back and forth if needed. Keep doing this until it turns decently (or at least a pretty good ok). Now, don't put the plugs back in. Put in your battery and hook up a charger. If the charger has "start mode", flip that on. Now turn the motor as though starting it, but only do 10 second bursts with about a minute inbetween to let the starter rest, and the wiring to cool.
If it doesn't turn, time to check wiring to the starter (usually a wiring to bendex issue for old Datsuns).
As you do this with the starter it will pump oil through the system (super important). After it's winding well, pop the valve cover and crank the engine again. Look to make sure oil is coming up to the valve train (or rocker assembly if a push rod engine).
Moves well enough? Well then... put the valve cover back on, hook up #1 plug to it's high tension and lay it on the valve cover on it's side. Crank and look for a spark. No spark? Well, then it's time to chase that back through the distributor, coil etc...
Got a spark? Excellent. Now put your plugs back in the head, and hook up the high tensions. Disconnect the fuel line to your carb(s), you don't want to suck crap into the float chambers.
Apply starting fluid & fire it up.
(if this is helpfull I can continue this further).
Here is a video of the first firing for my daughters car after we did all of the above. You can see by the look on her face, the suprise. You can also see the rear of my 70 in the garage behind it. All of the crap blowing out of the tail pipe is simply 10 years of sitting rust in the exhaust, made a nice mess. Roughly 48 hours before this video, the engine would have been pronounced deceased.
Once I sort out how to post images / videos I have plenty to share. For now, just a photo bucket link.