You can basically run a tape between the front and rear subframes and measure. Should have at least a 6-8" overlap on both the front and back subframes. The summit brand subframe connectors on my old car were nothing more than heavy square stock with a slight bend to it.
Whatever you do, you should have the subframe connectors welded, even if you buy bolt-in pieces, and then have an exhaust shop or someone weld them later. The bolts tend to elongate the holes in whatever they are bolted to and get loose over time.