The only problem I have seen is with performing "after the purchase" tuning on the E67 ECMs that come with those crate engines (apparently it is a custom OS). You may be able to find a much better deal on a used LS3 get an aftermarket harness and GM E38 ECM and call it a day for considerably less money. Albeit, it wont be new. Just my thoughts.
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I only wish they had the connect and cruise in 2010 when I bought my crate LS3 and 4L65E and ECU and TCU separate. With connect and cruise, the Transmission control harness plugs directly into the ECU harness without having to cut and splice wires to make a bulkhead connector. If I ever decide to do another LS swap, this is the way I'll do it.