Bellhousing to block interface is at the same point in the chassis and relative to the firewall, the V8 does not sit farther back than the staight six, so the same trans crossmenber is used with both transmissions, on either engine. From the back of the block to the transmission mount is the same distance on both the 3-speed manual and the TH-350 automatic.
The entire clutch/flywheel/bellhousing/transmission from the 250 will bolt right to the 350 V8, flywheel balance is the same, if you want to stay manual transmission.
You will need V8 block mounts for the truck to install the 350. Small block V8 bracketry for the power steering pump if you have power steering.