I have a magnum 25 single 454/volvo 280 outdrive. Just discovered that I have a noise in bottom end. I am looking to repower and was wondering if an outboard conversion(twin 200 hp )would be a viable option?

Yes, and allot of work! I'll sell you my 454 365 Mag, and just swap motors.

easiest thing to do is replace what is there. the hardest thing to do is replace the transom and put outboards on. if you don't already own 2 outboards, it will also be the most expensive. least expensive is most likely to be pulling your existing motor and seeing what is wrong and getting it rebuilt. it is possible, unlikely though, to be something with flywheel or coupler making the noise.

Sometimes a bottom end noise can be the Volvo intermediate bearing assembly that connects the engine to the drive. This can be fixed but you have to pull the engine & outdrive off to repair.. Some early Volvo's have only one bearing on the impute shaft and when they go bad they can make a heck of a noise.

that outdrive is bullet proof ,keep it.hill props in california is making a ss high performance prop this year for the 270 280 and 290,if you decide to get rid of it 280 let me no, i will buy it ,good luck

IMO . Modify the boat to outboards . . = depreciated value to a classic boat . . .
Plus adding twin outboards off the stern could drastically change the balance and performance . . .

Foillow up on H2Oboaters suggestion. These bearings are likely original and have never had the proper maintenance.
Start the engine and listen to the housing with a solid rod . . long screwdriver. You should hear only the whirring noise of a shaft turning in bearings . . I'll venture you hear a rumbling noise.

Mario L.