Welcome to the Stovebolt Dayton.
Can I restate your question to be sure I understand? You want to replace the original motor with a 350/350t. You want to change the gears in the rear, presumably to something more highway friendly. You do not want to change the brakes or suspension.
Did I get that right? If so, yes you can make these changes with out changing the suspension or brakes. You will have an issue with the steering, but it can be addressed without loosing the stock steering wheel. The existing steering box will interfere with the driver's side exhaust manifold.
You don't state your purpose. It sounds like you are trying to update while retaining a stock look. It might be easier to swap to a newer six, like a 292, which would fit in the engine compartment with out changing the steering.
Unless you're building a hot rod the 292 will drive as nice as the 350.
If you are building a hot rod I think you want to reconsider your commitment to keeping the stock brakes.
