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Hello, I have the stock original 289 cui in my 65' Mustang and I saw this one guy with a 1966 Ford Fairlane GT 390 and it had a 390cui block in it, also original to the car. The guy gave me a ride in it and I really liked the power that it developed for not too big of an engine. So I am curious as to if a 390 cui engine will fit in my 1965 Mustang and if not then what is the biggest that I should go?
 

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To answer your question, No, it won't fit, the Mustang engine bay was widened in 1967 just to fit the S code 390 in and that made it possible to install it's big brother the 428 in the 1967 Shelby GT500.

As to the stock 65/66 engine bay, the largest that will fit is the 351W and with stoker kits made today, it can be made into a 408 ci and with a special block, even a 427 ci
 

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Derek; Classic M is right,a big block won't fit. The 302 /351 engines are the best way to go. If the 289 in the car is original ,take it out and save it. The car will be worth more if you have the original engine to go with it. Pick up another motor and build it the way you want it. There are all kinds of performance parts available for small blocks. One thing to keep in mind; in 1965, Ford changed engine blocks. Early 289's used a 5 bolt bellhousing, late ones used a 6 bolt. All 302's & 351's are 6 bolt. If you have a 5 bolt, you will also have to change the bellhousing or transmission.
 

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Derek,

First, you should be checking into one of the nine buzillion classic Mustang forums out there. You will find the answer to your question and more in a place like that than here.

IMHO:

You have an original classic Mustang. It is your car and you can do whatever you want to it ...paint it pink with polka dots ....but you probably wouldn't. You could go through the swap but at least keep the original 289 ....because when the day comes when you need or want to sell the car, your top dollar is going to come from having it original ....not in some bastardized configuration. If you have the original engine, then any potential buyer can restore the car to its as-built condition.
 

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I would keep it all stock. The car is a classic as it is. You don't need the added power, unless you plan on taking it to the track.

Buy a fixer upper old car and hop it up.
 

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the 390 is an FE block their heavy and they love fuel.... you can just drop in a normal 302 their more efficient and parts more easily available or a 351w ooorrr a nice 428 fe block like the cobra jet
 

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o itd fit . u jus have to do sum hackin up an cuttin and weldin. my 65 is currently a race car i got for 250 wit no floor pan or gas tank and i have the 351w in it wit a bigger block
 
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