I believe it's 20 1/2" but it's hard to get an exact measurement with the v-bands on. LMK.
I measured 20 1/8 end to end on the Eurojet midpipe.
Here's my original for sale thread on it. I don't know if yours has the inside flange on it or not, but if not then this may actually work for you. This section doesn't have that flange on either side, but if you look at the picture in my FS thread, the top picture, you can see inside the Downpipe the "receiving end" for the flange is short, which is why it doesn't work with Eurojet Gen 2 systems (shown below with the longer inside flange). The 1st gen system has much shorter flanges.
The inside flange I'm talking about is on the right side of this (sticking out about 1/2 in or so):
It looks like from all the pictures I've seen online that the SPM system doesn't have that inside flange, but it also looks like that exhaust system is silver, whereas my midpipe is a bronze color. If that doesn't bother you (who's going to see it anyways right?) just send me an offer and we can try to work something out.