Nice truck! The 400's don't have near the issues that people claim and are a nice small block to build...... if you want a Gen 1, lol. That said if originality is of concern over performance a 283 would do that for you.

 

That tailgate is priceless.. you could get a good pic from it to frame... I would.

 

My Dad had a 58 that had a 327 and 3 speed on the column. Great truck. Wish I owned it now.