SL6 will not start with the key.

Valves don't just get "stuck in the guide." There is a reason. Either the valve is/ got bent, or whoever rebuilt the head did something wrong, like the valve guide is too tight.

I'd be 'fer yankin the head and inspecting it. Maybe that pushrod/ rocker was never seated right, maybe the valve contacted the piston and bent it, I donnoo. But I can tell you this:

I'd be pullin' the head off