There is no way your are building too much PRESSURE to cause a problem. A cap like this will allow some pressure and some vacuum in the tank - but in 170,000 miles on VO it has never been an issue for me.
UWS Replacement Fuel Cap for Transfer Tanks UWS Accessories and Parts UWSCAPS
Many do vent their VO tank to 'atmosphere' like timmy described, but this can increase condensation in the tank since as the tank cools - it draws in the moist overnite air. Some mitigate this problem by using a desiccant 'filter' on the vent line and placing the end of the hose in the engine compartment. (it will stay warm/dry under there longer than 'outside' the truck)
I suspect you have some other issue. Can you describe your conversion a little more?
What fuel pump(s) are you using?
What filter(s) are ya running?
How is your plumbing configured? (filter pre or post pump)
What kind of fuel lines?
What other heat do you have besides hotfox?
Sadly, most of the info on 'VO conversions' found on the web is poor at best...
Obviously you have a lot of time (and $$$) invested in this. Lets get it working right!!!
CC flatbed - headstuds, AC codes, T4/S366, custom leather interior, '07 facelift, big honkin bumper, 180k miles on DIY veggie conversion and a few other things...
, Limited, 4x4, BTS trans, 120k on veggie! - wrecked spring 2013 - RIP