The 345 is also a clamshell. To put any of the 350,353,346 top ends on it you need the riser block and the appropriate hardware. The 353 has a flat top piston the 350 has a dished so there's a noticeable difference in performance. The 350's also had open port top ends as well. The closed port 350/353 top end doesn't have limited performance gains they can be made pretty nasty keep an eye out for a thread in the next couple days.
Forgot about the different pistons in the 350 & 353 .
I have had several 350s,haven't had a open port one yet.
I will be watching !