subframe connectors - when?

If the engines weigh about the same, it shouldn't change the weight distribution much, if any, either. But the overall weight and distribution will start changing if you start upgrading the rest of the car to be more compatible with the V-8 - brakes, 904-727, bigger radiator, etc. Check the rest of the front & rear suspension parts for wear while you're at it

"When" you install the frame connectors shouldn't matter much, but "after" would be better. It's most important to keep the chassis and suspension level and properly "loaded" during installation. Welding them to or through the floor wherever possible will increase their effectiveness. If adding a roll bar too, it should be tied into the frame connectors, usually around the main hoop.

Welding along body panel seams is also helpful in stiffening the overall structure instead of relying on factory spot welds.