Reverse cooling

Your upper an 90s are likely holding air, no matter how high you jack the front end, they'll never burp. If those need to stay 90 degrees, I'd add bleeds to those connections or try and bleed by cracking those connections.

A higher pressure Rad cap will likely increase its "puking" because the difference between boiling at 12-18psi will be large and so the hot coolant goes from hot liquid to steam as soon as it gets past the rad cap spring. A high pressure cap helps prevent steam pockets in a std cooling system, but a well working reverse flown system should operate well with less pressure (6-8 psi, I would guess), especially if you're targeting less than 200deg F cyl head (coolant) temps.

With no flow restriction from the heads to the bottom of the rad, any hot spots or boiling are bound to try and lift the whole radiator capacity out of the cap when your pump is shut down. A standard system has a thermostat to overcome first and the hot water returns to the top of the rad, not the bottom, and so steam/bubbles have an easier time finding its way out. With your hot going to the bottom (I think?) that steam pressure tries to push the while rad out on its way to the cap..

Just my $.02, but I've admittedly no experience with reverse cooling..