It only needs ~40w from the 6-pin, so a dual-molex to 6-pin should work fine. I've used 110w GPUs on 250w PSUs before and had them work. Another time I tried the same GPU on a 270w PSU and it blew up.
In fact every time I have damaged something with an underpowered PSU it has only been the GPU that died. So you are probably only risking the GPU to be able to use it for 2-3 weeks. But if it dies, how long will it take you to save up for another one and a PSU?