There is plenty of room. The problem is going to be getting the GM ECU to talk to the Subaru ECU. It's possible, sure. Yet that solution posted above is by far the hardest thing I can think of to make for a Subaru, or for any modified car project. Back to reality, the only "good" solution I have found to this problem (and I have put very serious research into this) is to do the simple old cam trigger and crank trigger wheel/sensor faking. Feed the Subaru ECU just enough so it will a) crank the starter, and b) not throw a CEL code. The cam trigger is the key. You can clear away any other code, but you're not going anywhere without a cam signal unless you completely bypass every Subaru ECU function. Let's say you do that, no Subaru ECU outputs except CAN, no inputs except GM ECU's tach output (which has a different logic of trigger wheel shape)... you would have to find a way to get the cluster to come alive. Any way you cut it, short of hiring a software/hardware engineer, you may be best off just adding a crank trigger and cam trigger wheel in the Subaru format. If anyone has a plan that would work better, I would love to hear it.