I'd remove both hoses at the heater end and connect them together. That way you don't have to cut any hoses.
IF (big if) the hoses are different diameters, you should be able to find a 5/8" to 3/4" diameter adapter at your FLAPS.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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