small block grounds...?

Grounding to the K frame doesn't do a thing. The K frame is bolted to the WELDED unibody, all of which is one big piece, except for the front clip, doors and trunk. There is no other electrical "stuff" hooked to the K frame, so this is a dead end, and if the bolts get rusty, there you are.

You need the MAIN ground from the battery to the block, whatever you do. My front battery has the main to the block. I have a no4 starter cable (downtown store) from the rear head to the studs on the brake master mount, and a second no10 ground from the batt neg. to the fender

I used to run my Roadrunner, 440 6pak with a trunk mount bat. and that thing didn't have any trouble cranking, hot or cold. On it I had two "6 volt size" cables, store bought starter cables, grounded from the block to the unibody. This was 30 some years ago, but my recollection is to a bolted through hole on each side of the firewall