I have a 74...and one time i put a 289 Hi-Po in it...i found out that the 74 models were actually almost 3 inches shorter and 2 inches shallower in the front end...i had to move the radiater from its original position...take out the air conditioner...and put the radiater behind the grill and in front of its usual mounting place...that meant takeing the hood latch off,and going with hood pins...also had to pull the power steering as there was no way to get the cooler back on it...also the automatic transmission coolent lines ran right beside the exhaust pipes and i couldnt keep my
tranny cool enough to drive very long...there was just no space!!!
i have now replaced that 289 with a 2.8 ltr...and i am very very happy...its snappy,and gets 28 miles to the gallon..its no saturday night drag car...but its no dog either...
i bought a wrecked 75 model that had this engine in it...it had 173,000 miles on it...i have since driven it another 4 years and 30,000 miles and there is no oil burning,and all i have had to do is put one fuel pump on it...and that was a bear on the side of the road...but overall i am much happier with my 2.8 ltr than i was with the V-8 that was always needing something done to keep it going.
i wouldnt rebuild...just find you a good one from a junk yard...these engines are definately expensive to rebuild..aslo you will have it running faster that way...make sure the car you get your motor out of is junked because it was wrecked and not because of a blown motor problem.
1974 Mustang /Rebuild or get other 6 cylinder 
. The 74 stang only came with the 4 cylinder and the V6 engine so changing over to a V8 will require some fab work to get things to fit right, but it can be done. It all depends on what you want and what you want to do with the car.
