The cylinder and bolt hole spacing for a Ford 4.6 Modular engine is really close to that of a Slant. Over lay a 4.6 Head gasket over a Slant and you will see what I mean, not exact, but probably as close as one will get. This is a cross flow 4 valve OHC engine. So you won’t be messing with push rods, you will need to figure out a belt to turn the two front cam pulleys. Anyone going to this much effort would also go with a crank trigger and an external oil pump. The existing distributor and oil pump could be eliminated. Seems doable. Sectioning, adding spacers and re welding the v8 heads to make an I6 should not be a problem for anyone with a boat load of money.
So anyway, for a modern head that is really close to the slant six spacing, look at a Ford 4.6