I've been restoring a 78 for the last 10 months, just completing the front and rear suspension and PS work.  It has a TH350, but was originally a manual transmission car and has the welded crossmember (and an unemployed clutch pedal).  I had a 91 700r4 built for the car to make it more driveable with better gas mileage.  I'm not going to pull the engine so I'm looking at cutting the existing crossmember and welding plates on the stub ends then making or adapting a removeable crossmember to fit the longer transmission, something like the Bowtie Overdrive part.  I dont want to reinvent the wheel, so I'm wondering whether anyone here has done a swap like this requiring cutting and modifying the crossmember, and if so do you have any suggestions or photos you can share?  Thanks