All of the 5R parts will go on the old MBX5 no problem. If you do add the 5R/T shocks to the old car, you will need the 5R shock towers. Then you will have to either limit your droop or hit the arms and chassis with a dremel so that the lower shock end doesn't bind on the arm and the lower arm doesn't hit the chassis before the droop screw at full droop. Also you will need the 5R upper arms as with full droop the ackerman to steering rod connection will hit the upper arm if the wheels are turned at full droop. The new E0109B front uprights on the 5R also allow for more droop and steering so I recommend those regardless.
Best regards,