I would not count on it... the Manta/Ascona have the front anti-sway bar about 1' behind the front cross-member and the exhaust has to work over that. I'd be concerned that the flatter angle of the lower bends in the GT header would run straight into the anti-sway bar.
See post #25 in this thread to see side-by-side pix of the 2.
I have a GT header and will be dropping the Ascona exhaust and will make a quick test fit but it will be a while until I get around to it.
I need to correct my earlier post. I have one Manta header, from Calvin's motor purchase, and a couple of GT headers. I also have a Sprint, and 2.4l exhaust manifold. The headers, I will give to a new home, just pay the shipping. The others will fetch the current value price if purchased.