Thanks for the info.
Rather than weld weights onto the flex plate, I purchased a flex plate that is already weighted correctly for the crate engine (from B&M). Also required is a small bushing at the crank shaft and the torque converter.
The engine is in the car and will probably be running next week.
Most people think the 904 trans will work, but recommend that I beef up the rear end (it is now a 7 1/4). I have an 8 1/4 which will probably be fitted in the fall.