iMac 27" A1312 EMC 2429 GPU Downgrade?

Is it possible to download a graphics card in a iMac 27" A1312? It currently has a Radeon 6790M in, but has came in for repair, repair is for, it has pink lines down the screen and I have quoted the customer £235 ($364) for the part. This model card is much more expensive than this.

Edit : Would a AMD Radeon HD 6790 or HD 5750 be compatible?

Thanks

The Radeon 6790M is a "mobile" GPU which the 27" iMac uses. An iMac is in effect a large notebook computer built vertically. So no, I would not attempt to substitute another GPU which may or may not work right. The firmware likewise probably would not match the other GPUs.