In response to your original question, no, I don't see any problem with adding an overflow tank to your system is you're only using it to recover blown off coolant. The caveat that you need to keep the intake tube covered with coolant should go without saying.

My PV544 has a B20 engine and what appears to be an early, small radiator. The radiator is somewhat narrower than the frame it sits in, and doesn't have the 'roller blind' tube on it.

Anyway, the PO had continuous overheating problems, and replaced radiators and heater cores constantly. I think - from reading through the mountains of receipts - that her problem was a 15-lb pressure cap, not a design problem.

The system as I use it now (in high desert temps) uses a radiator cap with no pressure valve. The cooling system is totally unpressurized, and the overflow tank I installed holds 1/2 gallon. I installed a 6-bladed flex fan, replacing the stock Volvo 4-blade fan. That helped a lot, so I'm satisfied that my occasional overheating problem was due to poor air flow. The flex fan does not sit any closer to the radiator than the original fan, although it does come pretty close to the alternator bracket bolt on the engine side. A little grinding on the bolt head gave me plenty of clearance while still letting the bracket do its job.