This is really weird - Please advise

[62lwbc10]

Hello all, I have a 58 and I put a brand new gas tank in it, with a new sending unit and new filler unit, hoses, clamps etc. I usually dont put alot of gas in the tank, i put 5-6 gallons in, the gas gauge reads a little over 1/4 tank (I think its an 18 gallon tank, no?). Well I went to put about 10 gallons in the tank twice now over the past week and gas starts coming out as Im pumping gas. It seems like the filler is not adequately venting, although I have the vent hose hooked up. THis has happened twice. Can someone tell me how to troubleshoot this? ALso please confirm the stock tank is 18 gallons. I cant seem to get more than 12 or so in.

THanks!