Ron,
There were many different master cylinders used over the years. A picture would be helpful to see what you have. Many cars used Airheart with integral reservoirs and were later changed to Tilton with remote reservoirs. Very early cars used different off the shelf units as Tom mentioned. As another option, CNC master cylinders with the flat lid should fit under the floor (what I'm using) although the cars did vary quite a bit so your results may vary too
Jason