the way we used to do them years ago was pull the spring out of a regular springer,drop it with just the bushing in the shackle and cut the bolt so it doesnt hit you stem,knock the seat pan up from the bottom pan(can only do this with the older schwinn banana seats) get another bolt same size as the one on the springer shackle run that through the bottom of the pan,add some bushings from skateboard trucks,bolt it down and youve got a springer seat....if you want to 'tank it you can do it w/o welding....we used to get some old window screens cut them to fit around the frame to form a tank then bondo over the whole thing and sand it to shape(if your friend can really weld youre better off going the welding route though)...theres a few nice lowriders here just try using the search feature and you should find some of them
good luck
(just be warned they arent comfortable and are a bit squirrley to ride until you get used to them....also forgot to add use apes and rake them really far forward)