Box is strong enough for a 6cyl l series and Seidel engines. All depends how much power and how you drive it. Don't forget they are pushing 20+ years of abuse by now.

As for tunnel in an auto tunnel should be fine but manual people say needs massaging but I have a stanza tunnel in mine and still tight to get box out. Need to lift front of engine to get rear of engine down for enough angle to drop it out.

All depends how its mounted too.

What engine you running and what's your car now