IMO, only when 1) you can take the frustration that your car breaks down quite a bit; 2) you're very experienced in engine works (so you can do the swap, and more importantly, fix the car yourself when it breaks down), you should try to perform such a serious swap.
I've heard that there is a guy in Hong Kong successfully swapped 4G63T into his 6A13TT Galant VR4 (
http://auto.gznet.com/2006/2006-5-23/747938.html), so vice versa should also be feasible:
http://auto.gznet.com/2006/2006-5-23...8363825812.jpg
However, IMO, 4G63T is a better engine than 6A13TT, and you can also find much more aftermarket parts available to upgrade a 4G63T than a 6A13TT. So if you must do a swap, you should swap 4G63T.
Such a swap is a headache for even those very experienced mechanics. I suggest you to buy an Evo if you really want to go fast. I sent my Galant to UCS for 4G63 DOHC head swap, automatic to manual transmission swap, and customed piping for Evo turbo. It's been 11 weeks now and it may take another 11 weeks or so to get it done, because millions of minor problems occur. I hope this information can help you decide whether to do the swap or not.