question about minimum overlapping bases for oligomer annealing

[ut]

I want to generate a fragment  of 70bp  double strand DNA. The
stratage I choose is as followes:
Synthesize two oligomers  as short as possible with their 3' end
complementary with each other. Then  anneal these two oligomers by
means of those 3'-complementary parts. The rest of the gap will be
filled by klenow.  My question is how  many complementary  bases that
I should have at least  on  the two oligomers  are sufficient to allow
them anneal well and successfully  for  klenow filling in
subsequently? Is 6 bases long enough?  Your suggestion is appreciated
very much. Please send you suggestion to:
nianhua at utmmg.med.uth.tmc.edu