Actually just the opposite! Since the C's were larger cars, taller firewall, higher roof lines, the seats sat up much higher than the A, B/E seats. To make the C's more luxurious and roomy the tunnel in the C was made as small as it could be. Right up at the firewall the tunnel is going to be lower and probably no wider than your A tunnel. Since you have a C there really take a look at it and measure it, height/width and see for yourself.

What you could do is cut out the C trans hump and DS tunnel as much as you need and section it into your A body. It would take a bit of planning to make the cuts in the right shapes, wedges/triangles and tapered rectangles to fill in where you raised/widened your A tunnel but you would have the correct gauge metal. You could maybe just cut the top off you A tunnel and weld the C tunnel on top, spreading the A metal to get you extra height/width.

If you only need more room at the firewall, which it sounds like, you may only need to modify the firewall and a few inches back. JMHO

Cut the hole. Then Roll a piece of sheet metal over large PCV pipe Sticck it in from the bottom. Leave it long. Tack weld around the inside of the car with 1" welds. Go underneath and trim the excess. I welded mine from both sides completely so it could be metal finished. This is a B-Body but the same principle. Measure to the gas pedal first to leave room. The pice I used was small , thus the side seam. Works out great.
Doug