Which Melling oil pump should I use for a cam swap — 10% or 18% over stock pressure?

Pick by what changed in the engine. Summit's advice is to re-evaluate pump volume and pressure whenever you change oil demand — wider bearing clearances, high-flow lifters, spring oilers, an oil cooler, an accumulator or a turbo. The pump makes flow; the engine's resistance creates pressure; volume is set by rotor width and can't be adjusted.

Melling LS pumps:

• Melling M295: stock-replacement LS pump, standard volume/standard pressure.
• Melling 10295: standard volume, higher pressure (upgrade with optional pressure springs).
• Melling 10296: high volume, ~18% more flow than stock (~8 GPM vs ~6.8 GPM), ships with a stiffer spring (+10% max pressure / "+10 psi"), stock spring included; designed for single-roller timing chains (double roller may need timing-cover mods); for 1999–2016 4.8/5.3/5.7/6.0/6.2.
• Melling 10355: high-pressure pump for 2005–2017 Gen IV 5.3/6.0/6.2 with AFM and VVT (need higher pressure); +10 psi spring installed, stock and +20 psi springs included; 10355HV variant exists.

Most Melling performance LS pumps ship with a +10 psi spring and include optional springs, so pressure can be adjusted; volume can't.