700r4 drivehsaft question

[c20pickup]

after swapping my 700r4 for my th350 I have read I need to have my driveshaft shortened, in the future (before the truck is on the road) I plan on swapping the rear end, currently an Eaton, for a 14b ff or a Dana 60. am I able to have a one piece driveshaft built? im assuming with the new rear end and transmission id be better off building something from scratch instead of modifying my two piece. id rather eliminate the carrier bearing and the two piece driveshaft, has anyone done this? any advantages to the one piece? dis advantages? would the driveshaft angle be improved because the first contact point (uJoint) is farther away? (hopefully that made sense). some help would be great. im assuming id have to modify the cross member in order to run the one piece. the truck is a 1970 c20 lwb with a mildly built 350.