It usually had a V8, 289 cu. in. It could have had a straight 6, 200 cu. in. It NEVER had a 4 cylinder engine back then.
I had a 65 Mustang convertable that had a straight 6, 200 cu. in. and a Ford Anglia (English) 4 speed transmission with reverse lock out. It had so much torque that it could pull the wheels off the ground in first gear, could get tire in second. I could race the V8 289 and match it.
No V-6 , Not Back Then ,The Fast Back Mustang In 1965 Was A V-8 289 Over Head Valve Motor, (OHV), C.I.D. Displacement , in 1965 they also made a Fast Back Called A 289 2+2 High Performance Version, It Had A 289 V-8 Duel Exhaust Stick Shift Hypo Transmission, Locker Rear End And A Handling Package, Very Rare And If Found (Extremely Expensive). Today's Versions are, came from the same block with special updated modifications,same as the 5.0 or (302 C.I.D.)Today , But it Has Been Out Dated By The 4.6 Liter SOHC (Single Over Head Cam) V-8, & The Mach One 32 Valve V-8 Hemi,Also Today Ford Has Said The Future High Performance Motors Are To Be Over Head Cams Not Push Rod Motors , And Up Dated All The Important Vehicles, In The High Performance Direction, GM Stayed With The Push Rod And Ford Says They Are Old Teck , wich they are, Ford Is A Very Inovading Company, And Will Pull Out Of This Down Hill Sales Thing, All American Companys Are Hurting , Due To Forgine Imports, But The Tables Will turn They All Ways Do, So they Are Still King And Always Will,, The Only Four Cylinders, (In Line) were Used In The Ford Willis Jeep, That Was Used In The WWII War And Ford Made Them First Not Jeep,Great Question And Take Care, p/s back then you could special order it with a Inline 6 Cylinder, not a V-6, as well, but I don't think many fast backs had the 6 if any at all, but I have herd they were a few produced in 65, the first mustang rolled off the assybly line in 1964 1/2 and its in a museim its a white covertable, black interior and has never been driven in mint condition the serial # is 00001, V-6 Were found later in the more modern era in the Mustangs if not a GT Mustang Today, 2006 its a V-6 motor , and the did use the 4 cylinder in the 70's similar to the pinto motor was a 4 cylinder 1800cc and the 2000cc motor's but they made a few different sizes of the 4 cylinder and still do,
Remember folks that Tabby may just live overseas. Fords of England were always vastly different from those here. More often than not, they are a much much better car as Ford here makes cheap inexpensive junk to sell at a low low price. The only reason people buy them is because they cannot afford Chevrolet. So - In this case your answer may or may not be right. Did Ford put V-6 and I-4 into British Mustangs in the mid 60's. Check it out and let me know.
1965 Mustangs offered a choice of four different engines.
200 c.i. @ 120 h.p. w/9.2:1 compression and a one barrel carburetor.
289 c.i. @ 200 h.p. w/9.3:1 compression and two barrel carburetor.
289 c.i. @ 225 h.p. w/10.0:1 compression and a four barrel carburetor.
289 c.i. @ 271 h.p. w/10.5:1 compression and a four barrel carburetor.
Three body types available: 2 dr. hardtop, 2 dr. convertible and a 2 dr. fastback
A total of 559,451 Mustangs were produced in 1965.