My truck's 302 had a 1 1/2" block heater (for the freeze plug). Is that the same on the 351 blocks? My 393 stroker is coming and I'd like to order two block heaters for it and am wondering if they're still 1 1/2".
Freeze plug block heaters were a option, a rare one at that. I doubt the block will have it & it's a pain to tap them with the engine together. If you do tap them be very careful not to run the tap into the back of the cylinder, it will gouge them easily. I use one tap to start them and another tap I cut off to set the depth.
One heater is more than enough if it is around 850 Watts. You could even go to a 1000 watt. Just make sure your timer and circuit that you are connecting to can handle the load. That's about 15-20 AMPS.
850 Watts will warm a 5.0 to above 100 degrees in about 30 minutes in 25-32 outside temps. 5.8 should be about the same. A 393 wouldn't take much more.
Most diesels don't even use more than one heater.
If you don't want to tap the block, KATS makes a TEE connector that will splice into the lower radiator hose in lieu of the block connector. Inspect for leaks.
Tank type heaters is what I use.
It's a jeg's part number 555-51120....check out their description if yo'ud like. I had that same heater on my 302 which came on it stock (unless the previous owner did it but I doubt it). It was so so at best...took 2-3 hours to heat up totally. I don't think it runs the wattage you said it might....i could be wrong but I don't think it uses 850 watts. Keep in mind that I am not going to use both block heaters all the time...but my truck is subjected to temps below -30 at times. I called the engine company today and they said it's pretty easy to take the plugs out so when I get the engine I'll remove the plugs. It appears that they're all 1 1/2" so that'll work out nicely. Those style heaters are what I prefer.