Water pump to big
I ran the later water pump on the 340 in my 65 Barracuda for a while, along with a Duster 340 radiator. Fit OK, but no room for a clutch type fan. Was told by one outfit that my original radiator's top tank was rotten and it could not be repaired. After running the late setup for a while, I tried another radiator shop and they repaired my original radiator, put in a new three row core while they were at it. Then I put the short iron water pump back on. Kept the late timing cover for a while, using the early style bolt-on timing tab. When I got around to it, I replaced the late timing cover with the original 273 timing cover (I save my old stuff; sometimes it comes in handy). Also running the original 273 pulleys, brackets and crank damper, with a later A body clutch fan.
If there's no room for a fan with your radiator, it's in the wrong place. If you want to keep everything else as is, I'd modify the mounting brackets to move it forward. Then you can keep the late pump, just have to run a solid fan with no spacer.
Here's the short water pump setup in my 65:
I ran the later water pump on the 340 in my 65 Barracuda for a while, along with a Duster 340 radiator. Fit OK, but no room for a clutch type fan. Was told by one outfit that my original radiator's top tank was rotten and it could not be repaired. After running the late setup for a while, I tried another radiator shop and they repaired my original radiator, put in a new three row core while they were at it. Then I put the short iron water pump back on. Kept the late timing cover for a while, using the early style bolt-on timing tab. When I got around to it, I replaced the late timing cover with the original 273 timing cover (I save my old stuff; sometimes it comes in handy). Also running the original 273 pulleys, brackets and crank damper, with a later A body clutch fan.
If there's no room for a fan with your radiator, it's in the wrong place. If you want to keep everything else as is, I'd modify the mounting brackets to move it forward. Then you can keep the late pump, just have to run a solid fan with no spacer.
Here's the short water pump setup in my 65:
