Since you'll be limited to the 6-7 speed freewheels, you won't want to go to a wide range freewheel because the spacing gets too wide. The best option is to go with a triple crankset (need to replace bottom bracket also), to get the range you need without sacrificing spacing. Odds are you'll need a new front derailleur also, but those are cheap.
There probably will be some issues of RD capacity, but these are surmountable if you accept that your granny can only be used with the larger few rear sprockets. (be sure the chain is long enough to use the two biggest together even if you never intend to do so).
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