Yes, the variac and the bridge rectifier "would" work - but would
rather go the PWM route as I believe you can get better torque from
the motor at low speeds, and thats where I want to run it so I can do
thread cutting on my 9 by 20 lathe .

Essentially you are trying to design a well-regulated, high-current, DC
power supply with adjustable output voltage. You can cheaply get either DC
voltage regulation or unregulated high DC current; getting both is not
cheap and why commercial speed controllers are expensive.

My theoretical understanding would be that the Variac would be better at
supplying arbitrary torque (DC current) to maintain a set speed (DC
voltage). Variacs are "gutsy" (very VERY low impedance, a benefit of the
autotransformer design, maintaining steady output voltage irrespective of
current draw, that is, nonlinear and self-regulating). On the other hand,
you get some voltage droop with an unregulated bridge rectifier with
capacitor. But a PWM MOSFET or SCR arrangement is even less self-
regulating, depending on the impedance of the supply backing it. But then
SCRs directly on the line are the ultimate low-impedance supply.

It's possible to make a cheap speed controller from a few parts, it just
won't have the torque-independent speed regulation of the expensive
commerical item.