Stainless steel has better heat resistance than cast iron exhaust manifolds.
Aftermarket cast turbo manifolds are made from an alloy that has good high temperature characteristics.
The turbocharger is driven by forward pressure in front of the exhaust pressure pulse.
Any pack pressure behind the pulse decreases engine performance.
A good test to see if the manifold runners create scavenging in the other runners is to blow air into each runner and see if a vacuum is created in the other runners
Your log manifold doesn't have any runner length so there is no cylinder isolation, and each cylinder port sees maximum exhaust pressure from the other cylinders.
If the pressure from one cylinder is directed into the exhaust port of another cylinder, it will drive the intake charge back into the intake manifold when the intake valve opens.
I've had the plastic tips on the injectors melted from exhaust reversion into the intake manifold runners.
I would use your stainless shorties.
As long as the manifolds are not supporting the weight of the turbocharger, they will last.
Use a heat wrap or ceramic coat. The wrap or coating will reduce engine compartment heat as well as preserve a good turbo response.