As others have said you can bolt the 350 to a bellhousing that will bolt to the SM420. The original bellhousing will not fit the motor. The trick is you need to bolt that bellhousing to your frame.
Either you need to change the crossmember or make adapters to mount the bellhousing to the exisitng crossmember.
This is a doable project but it does require some work and maybe fabrication. I've seen it done very neatly, almost stock looking, and I've seen it cobbled together looking dangerous to drive or ride in.
