Exhaust seats, stainless valves, heads...

So I noticed on the previos thread that xxwelderxx was advised to change out his exhaust seats and valves. I'm pretty sure that I want to do the same for my 255 merc motor, but I'm wondering if at that point, should I just have the motor rebuilt??? Is it possible and/or even worth it to swap the valves and exhaust seats out without rebuilduing the motor??? I've never replaced either of those parts so I don't know how to do it or how difficult it is. Also, I was eventually planning on upgrading my intake manifold and heads. I wanted to slap on a set offenhauser heads, but I'm not sure if they will work if I'm running a stock bore and stroke. I know that edelbrock makes heads that will work with a stock set up, but I'm not sure if I want to go that route. As far as the intake manifold, I want to go with a Baron racing PM-7 Thikstun dual intake manifold with two stomberg 97s(http://www.baronracingequipment.com/...lManifold.htm). Does anyone know if that combination of the baron intake maifold and offy heads will work well with a stock long block? I guess that my goal for right now is just to pep up the asthetics in the motor compartment and I'm not building a race motor wich is why I'm slightly reluctant to start throwing together parts that I'm not sure will work well together. **) Any advice would be appreciated. Thanks!

Since no one has replied to your question I'll throw in my two cents. Hardened valve seats aren't necessary. As far as the rest of the thing you want to do, all are beneficial but only if you replace the cam to take avantage of the upgrades. You can keep the stock bore and you already have the increased stroke with the 255 Merc engine. You might check into the Isky Max-1 cam shaft for a good streetable engine.

The heads and dual carbs on a 255 will work with the rest of the engine stock. On my 255, I lucked out and got one with the Canadian truck cam and high compression heads, so aluminum will just improve the looks.
Some flatties already have inserts, most factory rebuilds do.