Datsun Roadster Forum-311s.org

Getting a lot of smoke inside and coming from this grate. Could it be an exhaust leak?

could be if its filling the engine compartment..you should be able to see it with the hood propped though if it's that bad

Smoke coming out that air intake is coming from under your dash. It is the inlet for your vents and the only connection to the engine compartment is the drain which lets out down by the tranny. The smoke is burning electrical insulation. On a high windshield the most common cause is the fuse box. Once you check for melting insulation at the fuse box turn the key on for short periods of time while watching under the dash.

Heater core seeping and smokes off from the heat ?
also a thing

If electrical, it will have a distinct acrid smell. We called it "essence of ohmite" back in the day when electronics used a lot of power - like in a Roadster. We all know what exhaust fumes smell like

Yes, be sure of differentiate between smoke (burning) and steam (coolant leak). There is some vulnerable wiring and an amp meter that often acts as a fuse when too much power is fed to it or the main wiring from the alternator grounds out.

Noticed a lot of steam building up the the windshield. Replace heater core?

Could be, but it may be the hose. Only way to tell is to dig into it... see if you can find the source of the leak.