Any idea how much I can cut and or offset grind out of a m block crank? Can I get it to 2.086" to use w 6.657" rods? Or would there not be enough overlap and snap it?

 

And I know I'll be going through the hardened surface. Pricing out getting nitrided and or induction hardening.

 

302C/351W/351C/351M/400 all have a 2.311" rod journal size. If you use a common 2.00" journal diameter rod (ie aftermarket SBC) you can offset grind any of these cranks approximately .20". This would give your 400 a 4.20" stroke. BUT, before you do all that work, contact TMI and compare the prices between the two options. Option 1: Offset grind your crank, have it re-heat treated, buy new rods, new pistons, have it re-balanced. Option 2: buy a TMI 4.35" stroker crank kit that comes with new everything and is balanced. Option 1 is 429ci while Option 2 is 444ci.

 

That was a kit available a number of years ago from "Stroker kits"
About $2000 for the kit back then.
It was a 4.200" stroke kit.

Issues, rod is so short. Now you have counter weight clearancing to do.
When you cut the counter weights, now you will be chasing balancing around next. Short rod makes the piston long and heavy.

Stock crank is not hardened.