Hi fred
I have not seen a proper tutorial and there is not that much information online, aspecially regarding a more affordable approach...
It's down to opinion on what the 'right way' is. To do the best job the original holes in the later body need to be welded up. This obviously is difficult to reverse and makes the car technically non - original as well as requiring bodywork and paint.
The other approach is to temporarily remove the late bumpers and dress the car In a way which can be reversed...
The parts needed to do a full conversion add up and are quite expensive. However is say the parts availability seems just as good, even better in north America than it is In Europe...
From the online prices the steel front valence, plastic front grille, bumpers and overriders, brackets, lights, rear engine cover and side trims might add up to $1000+!
You might be lucky and find a car in a breakers...
Personally I run my x with no bumpers although I will eventually fit fibreglass 1300 items. I have a fibreglass front valence and generic mesh front grill...