I have pod'd 2 boats; 24 SeaRay with V8 I/O & 33 Chris with twin IB's. The Chris has hollow glass stringers so rot there was a non-issue but reenforcing the transom was a major issue as was patching all the holes, minor issue on the SeaRay. Make sure the transom/springers/deck are sound with a good surveyor; better yet find a boat that didn't use wood in the stringers.
For a V8 I/O you will be cutting the power plant weight in half or so & placing the new weight farther aft. The length the bracket extends back from the transom SHOULD be calculated as a function go boat length - on the Chris it is 4 feet. The Chris just got finished, so i have little experience. The SeaRay was INCREDIBLE; raised the stern, dropped the bow so the boat ran more parallel to the water. At 50mph it would skip across 2-3ft chop on inland waters. 250HP V8 to a 250HP 2 stroke Yamaha; gained 10MPH top speed.
Best advise you will get; in US dollars new OB's are close to $100 per HP. On an older boat you may well need to replace MUCH more than the power plant.
Make sure the boat podder can engineer as well as weld.
FYI I got the 250 OB used for a great deal. Didn't need that much power. If you are in the open ocean & running with LARGE swells it takes LOTS of power to climb up & over them - otherwise depending on how/where you boat get an engine that you can troll on as kickers are less than optimal for boat control in wind