i have a 69 f-250 flat bed the original straight six motor was bad so the previous owner put in a straight six from a 91 he didn't finish it but i did. the question i am asking is was the older motor more powerful or are they the same? there is no hookup on the motor for a fuel pump and i put a cheap electric pump from the wrecking yard on it to get it home. it runs, but overheats. i changed the thermostat and water pump and it has the original radiator i think. but someone said the fuel injection heads on the 91 do not flow as well. it also has the original intake and exhaust. i had to tinker with the carb linkage but that all works now, in fact the only part that doesn't work electrically is the horn and alternator but i found the problem with that (bad regulator) and bad wiring. but i am going to put a higher amp alt. with internal reg in it. that and some rust issues that i am learning about. not too shabby for free. lal.

 

I'm not a 300 expert, but I think they were basically the same over their lifetime. Mild changes and improvements made, but basically the same.

 

I've never owned an EFI inline six but from all the reading I've been doing on the forum, the EFI heads flow better than the non EFI heads. Hopefully someone with actual real world knowledge will chime in to confirm or clarify.

 

The EFI engine has a different style combustion chamber, but the issue is probably the water pump. That year would have used a serpentine belt, while your old truck would have a standard vee belt. If it has vee belts now, you need an older water pump since both designs spin opposite directions.
Does it have a spot for the center sump dipstick? I thought the eliminated that hole when they eliminated the fuel pump. A rear sump pan may rub the steering linkage on a 2wd truck.

 

How soon does it overheat? If the temperature doesn't level out at all and keeps right on climbing from the get go, under normal circumstanced I'd claim it to be a failed water pump. The later engines had serpentine water pumps with stamped steel and spot welded impellers that would occasionally become damaged or broken. The earlier engines had a pump with a more efficient and reliable cast impeller. That being said, you replaced the water pump. My next guess is a blockage somewhere in the radiator. If you have a temperature gun, this is easy to detect, but basically the top of the radiator should be hot and the bottom should be a more palatable temperature.

Compression, like any engine back in the day, went down a little bit every year until I believe 1973 where it stayed the same. The EFI engines have slightly higher compression though.

 

thanks for the info. it has a regular v belt because someone must have changed the harmonic balancer as well. i think the problem overheating problem is in the radiator because there is a big blob of jb weld in the center and no fan shroud. i was wondering if the radiator from a 91 would fit and an electric fan. it starts to heat up right away and the tempurature gauge climbs to the top within 2 or 3 minutes. the truck sat in the woods for about 10 yrs before i rescued it

 

If it V belt and you replaced it with a pump for a serpentine setup, the impeller is spinning the wrong way.