I have a f150 7700 with the triton v8, 4wd 300,000 miles. rear main seal is leaking. Thinking of trying to put 300 i6 in it.
Any thoughts?

 

Depending on how much time and $$$ you have to work with, then comes the electronics involved . Engine harness is different. ECM is different. will the engine bolt up to an in line six without some changing of flywheel housings?
With that many miles,,,, don't know what was done to present engine, without that info the cheapest thing to do might be to just replace the rear crank seal.

Too many unknowns to speculate. Maybe someone that has done it could provide some addl advice.

 

Myself, I would repair what you have or put another exact replacement engine from a boneyard in there. If you repair, I would also plasti gauge the bearings to see if they are worn out and put a new oil pump in while the pan is off.If the leak isn't severe, I would just run it as is and maintain proper oil level.
Putting a straight six in there would involve changing mounting points for engine mounts, electrical harnesses and a lot more even if it bolts to your bell housing. The 300 I think is more durable but a far cry in performance to your present V8 and could be a real dog when mated up to your present gearing setup.I have a 300 engine, it is a work engine, slow, gets poor mileage and you won't gain much if any in gas mileage between the two engines.
I don't think this project is worth the expense and effort for a 22 year old vehicle.Just my 2c.