Will a late Ford 400M truck engine fit in 68 f100 where 390 was?

No, a Ford 400M engine will not fit in a 1968 Ford F100 where a 390 was without significant modifications. While both are big-block Fords, the 400M has different dimensions, particularly in the bellhousing pattern and overall length. The 390 and 400M have different bellhousing bolt patterns. You'd need to change the bellhousing, possibly the transmission, and likely the motor mounts to make it work. Additionally, the 400M might be wider, potentially requiring modifications to the firewall or inner fenders. It's a substantial undertaking, far beyond a simple swap.