20 ft/lb sounds good for a 5/16 x 14 bolt.

Tighten the bolts in a pattern and use small steps. 5 then 10 ft/lb etc. Also dont forget to use something like Anti Sieze.

A fine thread bolt can take much more torque before stripping then a course threaded one simpily due to the thread count.

The engine mounting on a Troy tiller has been ok for many years! If the engines broke off, or fell off, or the bolts broke out, i don't think they would have sold as many as they did over time.
If you still feel uncomfortable with it, i think you could fabricate a 90 Degree angle plate, upon which the engine would sit, and you could run bolts up thru it into the engine base. And then fasten it to the mounting bracket. Completely un-necessary, but it might calm yer fears.
If you use new lock washers and bolts, just tighten the bolts tight! All your worry about torque is just worry. Tighten them tight, then check after the first running and tilling, and if they are loose, then tighten them up again. The bolts should stay tight. For installations like that, i never use a torque wrench.
Rustyj

The main engine support is from two sliders that work with the forward and reverse lever. Those four bolts simply hold the housing.

Measure the height on these sliders before you remove them.