Hi Alex, sorry about the confusion about when you wanted to make a start on this revamp, thought you were still waiting on parts.
Your fist job is to remove the selenium bridge rectifier and old rheostat from the plastic cover. Clean up the cover and fabricate a couple of L brackets to mount your heat sink. Then fit your new rectifier onto the heat sink. I found self taping screws worked very well into the aluminium heat sink. (see photo)
Second job is to disconnect the rheostat from the old selenium rectifier, give it a bit of a clean. If you are as obsessive as me you can give it a coat of high temperature spray paint (these get hot, exhaust pipe paint is ideal). Next is to refit it to the plastic cover and reconnect the wiring. You will need to fit new lugs to accommodate the larger screws in the new rectifier, use solder or quality crimp fittings (not the cheap sh1t they sell in auto centers), heat shrink is optional but I think it makes for a safer/more professional finish. The two wires connect to the side of the rectifier marked with a squiggle/sine wave symbol, the polarity does not matter for the rheostat. (see photo)
Third job is to remove the front plastic cover and clean up the larger rheostat found within, when you are done it should look like the photo.
Bob