YAY, for the first time i can give a good "VMFer" response!
Well, there are some slight differences between the two - and here they are
First the 260 has a slightly larger intake at the lifter (being 0.2375) than the 289 - but the HP289 was much larger..around 0.28xx). The exhaust lifts are the same for both.
The crankshaft casings are identical until 1968.
The cylinder block bore is 3.80 for the 260, and 4.00 for the 289. Note, the 260 CANNOT be bored out to accept heads from a 289 because the walls are too thin.
Intake valves are the same for both
The "1964" 289 had a 4v carb, unlike the 260 which has a 2v.
The combustion chamber in the 260 (EARLY) is around 45cc, whereas the 289 is 54.5. Some people (rumors) say that VERY EARLY 289s had less than 54.5cc...but I dunno.
The 45cc combustion (260) ratio to head is 9.9:1
the 54.5cc combustion (260) ratio to head is 8.7:1
The 289 - many early versions has over 10.x:1 for its 2v versions, and its 4v version is 11.5:1
For the 260-289 and even HP289, connecting rods and crankshafts are identical
The 260, 289, 289hp, and 302 blocks are IDENTICAL to the eye - would have some trouble determining the differences without having model numbers/uncover valve covers..etc.
As a general overview, the 260 cannot be bored out to take the 289 pistons - which is a fact many mistake here on the VMF - many people told me to put 289 pistons on my 260 - cant do it due to the 260s thin walls. The 289 has a 4 inch bore (0.20 greater than the 260) - thus has a higher compression (over 10.00)
But, for most who ask this question - the better one would be - "how can I make my 260 preform like a 289" - and the answers I have received have all been the same (cost effectivness):
1. Change out intake manifold with a 4v version
2. Slightly higher CFM rated carb
3. Change rear gearing
4. Upgrade cam
5. Better distributor, MSD ignition, starter coil...etc.
6. Dual exhaust conversion
7. Generator - alternator conversion (for 64 models)
8. radiator fan/shroud set up - may require new wiring
and thats about it.
Hope this was helpful
1965 302 - kinda running
1964 1/2 - basket case