I have a 65' Mustang that had a 289 then someone put a Inline in it.

Now I am looking to put either a 351 W or C in it.

Does anyone know what problems I might run into? Do I need to change the orginal C4 transmission and/or suspension?

Any advice or help would be greatly apperciated.

 

Now THAT'S unusual. If the car has had the whole change to 6 cyl steering, transmission, axle and brakes, it'll all have to be changed to V8.

Installing a 351W or 351C is practically a bolt-in, you need special headers but everything else is stock 65 V8. Oh- you'd need a "5.0 conversion" radiator if your V8 engine has the water pump inlet on the LH side.