383 Rev Limit question

I'd say that before going much further, like switching to 1.6 rockers, you're going to need to find out exactly what you got. First I'd be surprised that your friend would build this strip engine and stay with the small exhaust valves originally found on the 915's instead of fitting the 1.74's. This was usually the first change racers made to these heads and the 516's that also had the small valves. Next would be what type of pistons are in it, flat top, flat top with valve reliefs or raised dome and what is the deck height, zero deck or slightly below. These items will come into play when you measure your piston to valve clearance and therefore if you can go with 1.6 rockers. What rockers are in the engine now, good roller rockers will get you 10-20 HP over the stamped steel because the stamped steel were rarily 1.5 from the factory and were actually in the mid 1.4's which robbed you of valve lift.
As said before ditch the Weiand and if it's a strickly strip engine go with the M1 or a little street the RPM Performer and that 850 is plenty of carb for that 383 maybe to much but time at the strip will tell. The Weiand has cast in restrictions on 4 of the 8 runners to make way for the manifold to head bolts. I got one for sale if you know of anyone that wants one.
Also if you're going to be spinning this thing at 6500 to 7000 rpm you're going to need a high volume oil pump and deep oil pan, 7 quart minimum.