Tom
Most of the data you need is in the article above as recommended by JohnV6.
Many +4s (especially the racers) upgrade the brakes by fitting the +8 calipers (which entails changing the caliper mounting bracket I seem to remember) which is an improvement. Also experiment with brake pads. The +4 brakes are good and using a fade resistant pad will to some extent compensate for the extra weight of the +8.
If you want to revert to standard +8 parts it could be an expensive journey, using the items from the later wire-wheeled cars. Your system can be made to work well as long as you are not expecting race performance.
