Ok, you've put up with my compression-ignitionist rantings for a while and have made it into the section that interests most people. There are some advantages to replacing your F series motor with a newer V8. The V8 weighs 250lb less than the F, which leads to a big improvement in your power to weight ratio. Because the newer motor is a little more effecient, and also because of the weight savings, you will get improved fuel economy. The engine that is most commonly swapped into a Land Cruiser is the venerable Chev 350. Other conversions, in approximate decreasing order of quantity are Chev 307, 383, 305, 400, 327, Ford 302/5.0l, Chev 454, Pontiac 455, Chev 4.3l V6. Swapping a Chev 235 I6 into a Cruiser is pointless because it is basically identical to an F. The Chev 292 may be a worthwhile swap for those who want that "authentic straight-six feel" Unfortunately, this engine is quite fuel hungry like the 2F and parts aren't nearly as common as for the 350.
Even though distributor placement at the front of Ford engines is probably better than the rear placement of Chev small block distributors, and the Ford 302 weighs 80lb. less than a 350, Chev engines are probably used ten times more frequently. The big reason is parts availability. While ford was wandering around in Windsor and Cleveland, the 350 changed very little over the years.
For that reason, a plain 350 is probably the easiest choice for an engine conversion. There are a variety of conversion kits available and the engine can be found in both carburated and fuel injected forms. For those looking for more low-end torque, a 383 is probably the best choice. A 383 is a 350 that's been bored .030 over and uses a 400 crank and 350 connecting rods. A 383 is superior to a 400 because the bore of the 400 is too large. There's no space left between the cylinders for water jackets so cooling is compromised.
There are only a couple of reasons for putting a motor bigger than a 383 or even building a high horsepower 350. They are if you intend to put on tires that are so large that re-gearing to return the tire:gear ratio to something approaching a stock level is impossible, if you drive your truck in deep mud, snow, or sand, or if you never quite managed to get that adolescent desire to try to peel your tires off your rims out of your system.
Key engine design features to keep in mind are the bore and stroke. Engines with a larger bore than stroke (oversquare) are better suited to high-RPM operation, while engines with a larger stroke than bore are better for lugging down at low RPMs. A longer stroke also allows for a lower compression ratio and lower octane fuel.
Once you have decided which engine you want to use, the next step is to choose an adapter type. Depending on the engine you have selected, you can either use a bellhousing from Advance Adapters, a Ranger torque splitter, or a Mark's Adapter.
In order to use an Advance Adapter bellhousing, the flywheel from the engine manufacturer must be used. In the case of small block engines, the manufacturer's heavier truck type flywheel is required and is advantageous because it will allow for smoother operation of the engine at lower revs. The advantage of using an Advance Adapter bellhousing is the low cost of the adapter itself. This savings leads to higher expenses elsewhere though. Because the Advance Adapter bellhousing is approximately the same thickness as the stock Toyota one, the engine, transmission, and transfercase will have to be shifted forward to all sufficient firewall clearance. The movement of the transfercase will require modified driveshafts. The use of the Advance Adapter bellhousing will also require a custom clutch only offered by Advance Adapters. It's probably best not to use a clutch that is not univerally available.
The use of a Ranger Torque Splitter provides several advantages. Foremost, you get a 27% overdrive for lower revs on the highway. You can use a stock Chev/Ford bellhousing and clutch. The Torque splitter functions as an adapter. it can be ordered with Chev or Ford bolt and inputshaft patterns on the front and Toyota input shaft patterns on the back. Finally, the 7-8" of extra thickness of the Ranger means that driveshaft modifications are not required. The Ranger is said to put the fan a little closer to the radiator than ideal though.
The third option is the Mark's adapter. Their kit consists of a bellhousing and flywheel that are 3-1/2" deeper than stock. The extra depth places the engine perfectly with no driveshaft modifications. The extra thick flywheel also provides extra damping to allow for smoothly lugging down the revs in the rocks.