The problem is not the crank but the chainring. If you cannot but a spacer behind the driveside BB cup to move it out a little bit, only a smaller chainring will help. Go to a shop and get them to install the BB (with a spacer) and a 39t ring. If it fits, buy it from them. If it doesn't, offer to trade them the crank you have for one that will fit.

Also, you'll probably need to swap the 18t cog you bought for a 16t or so, to compensate.

Edit: Ask to trade your S300 for a Truvative Stylo OCT 1.1G. Keep it in the Sramily and it should fit better. Alternatively, there are a billion others to choose from that are all worth their salt.

But seriously, consider the hammer if the frame's not worth a ton of money. My city bike wouldn't fit a 39t ring. Now it does.