It's your car so you should be able to do whatever you want but personally if it is a real big block car I would think twice before cutting it. I agree that a manual trans is a lot of fun. Both my cars are stick. You could always go with a 4 speed overdrive setup with better internals and I've heard if you replace the aluminum side cover with a cast iron one it is more durable. Maybe someone else could chime in on this. If you not racing at the track a lot I don't think the 4 speed overdrive would be a detriment and it would be much better on a long drive.

 

Of course cutting hurts the value.
But your car really only has to make one person happy. You.

 

One of 701....I have one, 383 auto....I wouldn't...but your car...if you do, save the metal you cut out and cut it out in a way it could be put back....just my opinion

 

I had the same dilima, I have a 68 383 suto and I changed it to a factory correct looking 4 speed set up. I kept the tunnel cut and shifter set up that could go with the car if the next buyer wants to convert it back to original. I really enjoy the 4 speed and from the people that I have talked to I don't think it hurt desireablility at all maybe made it better. If it was a low mileage documented cream puff probably not, but a driver I say do what you like. At this day and age of 60's muscle cars I don't think it matters. Enjoy them to the max, the next generation will only see them in games on their cell phone.