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