Dear Modiol;
You may do it either way, with or without the pressure cap. If you run W/O pressure the fluid will seek It's own level in the top tank and not drop any further. This will be about 'half full' which is just fine.
The system will hold what It's designed for (0-4#) and probaly 7 without any problems. If (that word again) everything else is in good condition etc.
Head gaskets "blow" for OTHER reasons.
Almost all 40 vehicles vehicles ran W/O pressure in the cooling system and if It's not overheating, all is well.
"Don't worry be happy".
