The /ʌ/ vowel is spelled with <u> and <o> in ton. The /aʊ/ vowel is spelled <ou> or <ow>.
This is a contrast between a short vowel and a diphthong with a similar starting position but distinguished by lip-rounding. It does not seem to be a problem for many learners.
There is one taboo word pair: cunt/count.
Interesting pairs include:
curry cowrie
stutter stouter
The density figure is 4.31%. The set makes 41 semantic contrasts giving a loading of 44.6%.
abut about bud bowed but bout butt bout butts bouts buzz boughs crutch crouch crutches crouches cud cowed cull cowl culling cowling culled cowled culls cowls cunt count cunts counts curry cowrie curries cowries does dhows done down dost doused dun down dunned downed dunning downing duns downs fund found funded founded funding founding funds founds gun gown gunned gowned gunning gowning guns gowns gut gout hull howl hulled howled hulling howling hulls howls just joust lunge lounge lunged lounged lunges lounges lunging lounging mud mowed muss mouse mussed moused musses mouses mussing mousing none noun nun noun nuns nouns nut knout nuts knouts punned pound putt pout putted pouted putting pouting putts pouts ruddy rowdy ruddier rowdier ruddiest rowdiest rut rout ruts routs rutted routed rutting routing scull scowl sculled scowled sculling scowling sculls scowls scut scout scuts scouts shut shout shuts shouts shutting shouting stutter stouter sunned sound suss souse sussed soused susses souses sussing sousing ton town tons towns tun town tuns towns tunny townee tunnies townees tut tout tuts touts tutted touted tutting touting utmost outmost utter outer uttermost outermost
John Higgins, Shaftesbury, December 2009