I had a stock L18 in my 510 with a Weber and 5 speed. It was nothing special. Just a daily driver for a couple of years. The engine mounts and tunnel conversion for a bigger motor and tranny can be tricky. See the first page of my Z24 conversion. I built the motor mounts right on the motor in the car. Key is to lift the car up and set the tires on blocks. Get the full weight of the car on the tires up off the floor for good working clearance. Slip a thin piece of blocking on the cross member. Drop the engine and tranny in and set the oil pan on the cross member. Tie wire the rear of the tranny up through the shift hole and wrap a piece of wood up on top of the tunnel. Now you have full weight of the engine on the car and and on the tires. Move engine into the position you want and weld engine and tranny mounts. Make sure hood clears.
That's how I did it. It is a bit of work though.