Initial baseline for timing

Magnum heads do not require as much total timing as open chamber LA heads so your 34 degrees may be a little too much for peak power. You may find that 32 or 33 may make more power but 34 is certainly a good starting point. On the opposite end the 18 may be a little short for the initial but again is a good starting point.

Assuming you have decent traction with the gears and convertor you should be able to get into the 12's on motor only so 12.40's with a 100 shot is quite reasonable.

FWIW, my 360 with magnum heads, 10.6:1 compression, XE268H cam (224/230@050, .509/.512) and 670 Holley is making approximately 370 HP (this is based on the car running 102+ mph in the 1/4). I would say your engine is close to that figure to. The MP 360/380 engines typically dyno in the 410+ HP range.