drill  a hole in the steel plate and use a 3mm nut and bolt - that's why I suggest using an insulated tab type device as you don't have to be too fussy about mounting - just bolt it down.

Forget about specs like gate current etc, in the "olden days" components were always running close to the edge spec wise but most modern parts are MUCH tougher than those used originally. The one I have suggested you use as a replacement is physically 1/4 the size of the original but will carry x10 the voltage and current and is probably half the price the original was all those years ago.

This is a very NON critical application for this device and almost any triac will do the job equally well just make sure you Google the pinouts of the original and the replacement DO NOT assume the pinouts will be the same.