vacuum reference port

I would not do that. Use a larger return, and you really should use at least same size as supply. My 67 of course had no 1/4 vapor line. I brazed a 1/4 line into the tank which was fine for a vapor return, then later converted to EFI. Even with a "low" output TBI, the 1/4" had lots of back pressure. I managed to 'get by' using 3/8 all the way to the tank, than adapted to the 1/4" nipple. There is not enough restriction in a few inches of 1/4" tube to matter at this time. If I get more power in the thing, I might have to rework that fitting.

Also are you aware that your 1/4 factory line IS THE VENT. You will have to provide something else for the vent