Also is it possible that I could just have some custom made and around what would something like that cost?
Also what are header flanges? they look as though they bolt straight to the head, but are cut extremely short, is this an adapter and for what type or problem would this work?
long tubes are out of the equation,it will have to be shorties unless things have changed since I had a 78 that i swapped a 351w in a few years back,no one makes long tubes for this truck/engine combo. I used MAC mustang shorty headers that I got off of ebay. the driver's side fit like it was made for the truck,the passenger's side had to have some tricky bends done to clear the frame,but it is possible. you might try to contact a user on here by the name loudfords,he has some pictures of the shorties on his truck.
Shorties will indeed be fittable, but there will be minimal power gain. Exhaust all possibilities for finding small tube, full length headers before resorting to shorties. Full length headers will give you SIGNIFICANT torque gain throughout the rev range, from idle up.