That is the same part number I found for that size. Where did you find the info that it says it is actually for a 1978 FJ40? I'll probably go ahead and give it a try.
They used to have a page that let you select your vehicle and it would give you all the products they made. This is the list for a number of years including a '78, but I cannot vouch for their accuracy. I know that when posted all the tabs/spaces will disappear so I apologize for the display you see.
Part No. Product Line Part Qualifier
BR50 Bearing-Wheel Front Wheel 4 W/D; Outer
BR50 Bearing-Wheel Rear Wheel; Outer; W/Full-Floating Axle
GRW146 Bearing-Wheel Rear Wheel; Exc. Full-Floating Axle
JLM104910 Bearing-Wheel Front Wheel 4 W/D; Inner; Cup
JLM104910 Bearing-Wheel Rear Wheel; Inner; W/Full-Floating Axle; Cup
JLM104948 Bearing-Wheel Front Wheel 4 W/D; Inner; Cone
JLM104948 Bearing-Wheel Rear Wheel; Inner; W/Full-Floating Axle; Cone
13857 Oil Seal-Axle Shaft Front Wheel 4 W/D
13911 Oil Seal-Wheel Rear Wheel; Outer; W/Full-Floating Axle
18695 Oil Seal-Wheel Rear Wheel; Exc. Full-Floating Axle
24635 Oil Seal-Wheel Front Wheel 4 W/D
36655 Oil Seal-Wheel Rear Wheel; Inner; W/Full-Floating Axle
N3075 Bearing-Clutch Release Bearing Only
6202-2RSJ Clutch Pilot Bearing/Bushing Bearing
KC11445-Y Bearing-Differential-Axle Front Axle
KC11445-Y Bearing-Differential-Axle Rear Axle
14767 Oil Seal-Pinion Front Axle;Wagon
14767 Oil Seal-Pinion Rear Axle; Wagon
99147 Oil Seal-Pinion Front Axle; Speedi-Sleeve; Wagon
99147 Oil Seal-Pinion Rear Axle; Rear Speedi-Sleeve; Wagon
6201-2RSJ Bearing-Alternator Commutator End W/12 Volt 40, 45 & 50 Amp
6203-2RSJ Bearing-Alternator Drive End W/12 Volt 40, 45 & 50 Amp
6305-2RSJ Bearing-Alternator Drive End W/24 Volt 40 Amp
39125 Oil Seal-Crankshaft Rear
99393 Oil Seal-Crankshaft Speedi-Sleeve
17758 Oil Seal-Timing Cover
99177 Oil Seal-Timing Cover Speedi-Sleeve
6203-J Bearing-Steering Pump Shaft Power
6207-J Bearing-Input Shaft W/ Manual Transmission
BR30307 Bearing-Output Shaft Rear Output; Front; W/ Manual Transmission
BR30307 Bearing-Output Shaft Rear Output; Rear; W/ Manual Transmission
BR63 Bearing-Output Shaft Front Output; W/ Automatic Transmission
18962 Oil Seal-Output Shaft Rear Output; W/ Manual Transmission
18963 Oil Seal-Output Shaft Rear Output; Polyacrylate; W/Manual Trans
18973 Oil Seal-Output Shaft Front Output; W/ Manual Transmission
99189 Oil Seal-Output Shaft Front; Speedi-Sleeve; W/ Manual Trans
99189 Oil Seal-Output Shaft Rear; Speedi-Sleeve; W/ Manual Trans
18987 Oil Seal-Side Gear Seal; Wagon. W/ Manual Transmission
4707 Oil Seal-Shift Shaft
99049 Oil Seal-Shift Shaft Speedi-Sleeve
11136 Oil Seal-Front
12735 Oil Seal-Front Wagon
99112 Oil Seal-Front Speedi-Sleeve
99128 Oil Seal-Front Speedi-Sleeve; Wagon