i would probaly make a 351W with 351C heads on it just for a cool combo. they can be done pretty cheaply too.

 

390, 406, 410, 428, or anywhere in between there. Get some torque []

 

I'd hate to see you change the motor in what sounds to be a pretty original car. You can make it FAST by putting in a big, really modded V8. But it will be and feel heavy in front, it will not be a really balanced car, and if you stick with a carburated version (most do) it will get poor fuel encomy and only so-so driveability. On the other hand, very little has as much class as a well turned out 68 Mustang that hasn't been screwed up with mods - a true classic.You can buy a lot of chrome parts to dress it up, and performance parts to boost output quite a bit while keeping that 289. It will be faster, but never FAST compared to the really quick street cars on this or similar forums.

If you are going to change the engine, while you can go with big block engines, a 302 or 331 stroked 302 would be a best choice. Its basically a derivative of the 289 and weighs about the same, fits right in, and feels, in every way, like its meant for your car (because it sort of is). I'd get a fuel injected/digital ignition one, for sure, which might mean changing fuel pumps (I'm not sure). Easiest and cleanest way would be to buy a fuel crate engine, with engine computer for the fulel injection and ignition, and then put it in with headers, etc. You'd double your HP, and you would additionally find plenty of chrome dress up parts for you engine available from numerous sources.

If you go with a bigger basic engine you have the potential for more HP but also to spend much more money and probably not make it as nice a daily driver.

 

Hey, I can tell you from exsperience the most efficient way to go would be storke the 289 out/347. Shop around educate yourself on what quality parts they are selling in there kits. Goin with larger displacement 351w/stroked out is fine. Really not much more for quality kit but when you go with windsor block/taller then 289/302 it is gonna give you less options for what you can use for header applications. Headers can be a bitch, what everyone will try to sell you and what will actually fit without gettin the hammer out. I got a stroked 351w/408 in a 67 fb , after a couple of months digging I finally settled on jba mid length headers for my car, exspensive $700 and chainge.When you pushed the subject with everyone no really guarentied installation with anything else.

 

i think someone already said it...but 390 would be a great torque monster....and if you have an extra 15k laying around...just go with the 427 sohc dual carbed motor.