For the most part I agree with all the comments...

A basic rebuild of your 326 vs a 400 or 455 build will probably cost more and you'll end up with a nice 250+ HP diver. If you are able to source a set of 141 heads you might be able to squeeze out 300 HP.

Muscle Car TV built up a 326 for a LeMans a couple of years ago. It was stroked to 376 inch and reused the 140 heads (could not find 141's). Not sure if it was dynoed, but it looked like it made good power. Still a lot of work and $ to make maybe 320 to 350 HP.

RossettiJ is right, nobody is clamoring for a 326 car. You wont loose anything with a 400 under the hood. That's what folks want and that's why original 326 cars are becoming even more scarce.

If it is important to you to keep it original then rebuild the 326. But regardless of what you decide, it is your car, so build it how you want it.

Personally I love my 326, but I'm a bit of a purest. It is what it is and it's only original once.