There are no problems with spelling since /p/ is always <p> or <pp>, while /Ɵ/ is always <th>..
This contrast is between a stop and a fricative. Both are both unvoiced but they are well separated in the mouth. Learners are unlikely to confuse them even if they have trouble make the /Ɵ/ sound.
Interesting pairs include:
epics ethicsThe mean density value is 1.6%. The list makes 70 semantic contrasts, giving a semantic loading of 54%.
beep Beith burp berth burps berths burped berthed burping berthing burp birth burps births cap Cath dep death epic ethic epics ethics grope growth gropes growths hap hath harp hearth harps hearths heap heath heaps heaths help health helpful healthful keep Keith kip kith lope loath maps maths mop moth mops moths neap 'neath pain thane pains thanes pane thane panes thanes patch thatch patched thatched patches thatches patching thatching pause thaws paw thaw paws thaws pawed thawed pawing thawing pawn thorn pawns thorns peeve thieve peeved thieved peeves thieves peeving thieving peevish thievish peevishly thievishly peri theory peris theories perm therm perms therms pews thews pi thigh pis thighs pick thick picker thicker picket thicket pickets thickets pie thigh pies thighs pin thin pinned thinned pinning thinning pins thins ping thing pings things pink think pinker thinker pinking thinking pinks thinks pip pith PO Theo pole thole poles tholes poll thole polls tholes pore thaw pored thawed pores thaws porn thorn port thought ports thoughts pour thaw poured thawed pours thaws pressure thresher pressures threshers price thrice pro throe pros throes pro throw pros throws prone throne prone thrown prong throng pronged thronged prongs throngs prose throes prose throws Pru threw Pru through pug thug pugs thugs pump thump pumped thumped pumping thumping pumps thumps purred third pursed thirst rape wraith rapes wraiths reap wreath rope wroth sheep sheath sloop sleuth sloops sleuths slope sloth slopes sloths soup sooth soups sooths troop truth troops truths trope troth troupe truth troupes truths whelp wealth whip withe whips withes
John Higgins, Shaftesbury, January 2010