How can you tell the difference in stock proportioning valves say disc rear verses drum rear, and can they be modified ?

There are board members who can probably be very specific about
the differences. Until they chime in, here's
Within the same model year, it's very difficult to determine by exterior
appearance. And having dis-assembled the disc & drum models for
my 90 formula, I can say that the difference is internal springs for the
hyd. piston(s). Beyond switching the springs, I wouldn't recommend trying
to modify these components. The most popular method is to switch to
an adjustable one for the rear (like SSBC) in combination w/ a tee fitting
for the front lines, which is what I'm currently in the middle of.
Total cost of materials (valve, fittings, etc.) will be in the $80 range.
Biggest hassle is lining up all the correct stuff. When I'm done (hopefully
soon), I intend to post some pics, & a summary of how I got there.