273 Commando with Hydraulic Camshaft

I have run a 340 cam in a HP 273 4 speed and it ran great. The first thing to do is check TDC. I have seen early LA engines be over 10 degrees off. Since you are running a 4 speed, 3.23 gears, and 25 inch OD tires you should be good there. If I had to pick a hydraulic cam I'd pick one similar to the old MOPAR Performance P4452759. 260/268 duration .430/.450 lift. 44 degree overlap and 110 c-line. Maybe a step lower. Use 340 valve springs or equivalent and a good true roller timing set. Centerline the cam. Your 920 heads are closed chamber and flow very well at low lift to mid .400's. Distributor is good, just make sure it is working as it should advance mechanical and vacuum, points set right.