Slow Crank, No Start When Warm

[1] If you are running a grd cable from the starter [ block ] to the battery, that will be part of the problem. Too much resistance, which causes a power loss. You should use the body/chassis as the grd connection.
[2] I had a look at the TuffStuff website & two starters are shown for the SBM, a 'std' & a more powerful one for high output engines. Neither one has a HP rating [ post #14 ] , which makes me suspicious that are low HP. A high HP rating would be a plus for selling the product. The starter needs to produce a certain amount of HP to crank the engine. When hot, the res increases & HP output is decreased. This loss has to be factored into the choice of starter used. That is why there are 0.9hp starters &....3.2 hp starters.
[3] The TS starters are the Nippon Denso style, motor on top. Never found them as powerful as the Hitachi design, sol on top.