5/8" or 3/4" lines will work. There is an arguement to be made that the amount of additional coolant capacity gained by using 3/4" lines doesn't negate the added "dwell" time spent in the line in close proximity to the engine heat. 3/4" lines are also more prone to leaking behind the SC where they slip onto the 3/8" nipples for the IC core. Leaks don't seem to happen very frequently but they have occurred.
I opted for 90 degree molded 5/8" ID lines from Napa (part # NBH 9814) so I wouldn't have to worry about kinks or Uni-Coils to prevent kinks. My decision to use the smaller 5/8" lines was influenced by the fact my heater core crapped out back in June while traveling cross-country and I spent way too much time and effort trying to keep the 3/4" line I used to bypass the heater core from leaking so I could reliably finish my trip.
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