I'd lean torwards the Mach 427 if the price is right. Excellent engine for a Savage. Just enough power to be super fun without destroying the truck's drivetrain every two gallons. The Go .25 will be hard if not impossible to start with a spin start backplate in a standard length Savage.
You'll need 16T CB and a 49 T spur for the Savage for the Mach 427 or the Mach .26.
The Mach .26 SS is not a good as the other Machs are, but it's not bad. It's more powerful than the Nitrostar .25 for sure. The nitro star .25 will be the best fitting engine listed here, but the least exciting. They are very reliable though.
BTW I have had all these engines except the Go .25 in a Savage, just trying to help you make a wise choice.