This doesn't much look like REXX to me. What model card punch do you have?
If you want to assign a string you just do so...
if u need it to be 1500 long then say
astring = LEFT(value,1500)
so what u could do is...
and then overlay the fields at the offsets you require?
astring = OVERLAY(block,astring,12,100)
astring = OVERLAY(clump,astring,256,10)
does that answer it?