The 63-69 cap is a vented cap so you'll be fine with this.
https://willcoxcorvette.com/corvette...p-vented-63-69

 

 

I believe those vented caps only allowed air into the tank to prevent vacuum. On my 74, my charcoal canister is long gone. I use a vented cap to handle vacuum and a "remote mount rollover vent valve" to handle over pressures. Some people have been known to just drill a small hole in their tank but gas will invariably slosh out.

 

My '72 set in the garage, next to the kitchen, wife has a sensitive sniffer for gasoline, ME not so much.....oh well, vette had an issue with tank, and the way to fix the problems is simple, really....number one is that I have a DPFI setup on my SBC..... so the fuel pump is in rear UNDER the tank, tied to frame rail, pushing fuel forward to rails, regulator goes back to tank up top on pass/RT side....just like it did with a carb decades ago....

BUT on top left/driver side is a black plastic float/check valve, takes the tank fitting and if fuel is present the float blocks the vent closed, that line goes forward to the charcoal canister , the canister has a manifold vacuum valve on it, pulling the discharge/vapor line from the canister open when engine is running, and that vent line goes to the PORTED vacuum opening on the throttle body....essentially same as with a carb, call it feeding into the inside of the air cleaner if you want, it's above the throttle blades.....

on my car I discovered a failure on the canister control valve, it was stuck OPEN for some odd reason....fine by me, I ran the rest of the lines per stock/usual and then got a two way vented cap on the tank.....

what WAS happening is the stupid stock cap was a closed type, here in FLORIDA the summer heat would combine with the heat of the returning fuel and cause a positive pressure in the tank, I could hear the air/fumes escaping when I went to fill the tank.....the sealing was not purrfect to the fumes would get into the garage air....

BUT with the float valve being not flooded/open, the fumes/pressures were sucked into the engine with the mild/ported vacuum, and PROBLEM SOLVED.....and so, logic tells ME, that running that line that went to the cannister, it can be run straight to the ported vacuum fitting and all should be fine....BUT that ASSumes the liquid check valve on the tank is operating correctly.....

 

 

Fix the float valve, and do the way I said above, or you will have gas fumes in the garage.....

 

 

I have a 74 also and had a problem with the canister couple years ago. I simply drilled a 3/32 inch hole in the gas cap and I have had no problems since. My car is kept in my garage adjoining our kitchen and have not had a hint of the smell of gas in the last 2 years.

 

Nope.. just in.. it won't allow the tank to vacuum lock. Honestly you don't need anything else if you use the vented cap. However the ideas above seem creative to me and the only issue I see with drilling a hole in the old cap would be that fumes could get out... On such a tiny hole, I'm not sure that it would matter..

Willcox