The high hump can also be from an automatic, my 72 has a TH350 and has one, it has a hole for the T case only, you could find one on the parts for sale board for probably about $40-$50.
I've seen the conversion on a thread here where yes they just lay it over the top and trace it, when you cut the old one out just measure back an inch to 1 1/4 and cut the low hump out, sand and paint the cut area, use some seam sealer when you screw the high hump into place.
Heres a post in my build thread showing my high hump is a bolt in,
http://67-72chevytrucks.com/vboard/s...&postcount=439