with stanza struts i just fit 15x6 +38 but they needed spacers
judging by spacers i have min on front would be 3mm
back i have 5mm but think i need more like 10mm
so with 6in width rim on front i would suggest +35 down to +25 on front, if the offset is higher you can use spacers to fit them
with 6.5in rim on rear i would suggest offset +26 down to +16 or even +10
back to the rims you propose..........
6in +20 on front would sit a fair way out from strut
6.5in +5 on rear would also sit a fair way out
should be ok with some vary minimal pumping of guards
my calculations are based on 195 tyres by the way, if you were running say 205 plus you would need the extra clearance for the tyre width... but your gonna have bigger problems again with getting them to fit inside the guards
hope all that can be understood
this helps....
wheel offset calculator