I need a heater core with 2 different hose sizes for my 68 Barracuda

FWIW, my '69 has uses 5/8" heater hose in and out.

From Mymopar '68 parts book...…….
upload_2018-12-10_11-6-43.png
upload_2018-12-10_11-0-19.png
upload_2018-12-10_11-5-22.png

1315466: 5/8 heater hose 50 foot roll. ss to 4741932AB $57.50

Parts still available from your mopar dealer