I have an 86 c-10 shortbed. I rebuilt the 350 with a .030 overbore, flatop pistons 9.5 compression, comp cam 268 high energy series, aftermarket heads. not sure what brand the heads are, all I know it has a logo with a D inside a circle., it has 2.02 1.60 valves, edelbrock performer manifold, edelbrock 600 cfm (1406) and some headers. my question is, what cranking psi should i expect, i get a reading of 150 psi, motor has not been on the street yet. i also am running 2 1/4 inch duals with flowmasters. should i step up to 2 1/2? What kind of power do you think i am making? is my cam to big for my application.  it has a th-350 with 2.73 rearend . i am planning to step up to 3.27 or lower. it runs good except when i get the vacuum reading the needle jumps from 16-13 in-hg back and forth. there seems to be no vacuum leaks anywhere. any suggestions i would really appreciate. i am only 16 but am doing all the work after school including body work  

150psi is fine the cam isnt to big so the motor runs good your just conserned about the vacuum ? had the same problem with mine the dual plane intake put a one inch open spacer under the carb try that   my vacuum gauge is hooked up to the intake thow did you just test it at the carb .  the exaust is ok  

the HE268 is a good cam and should provide ample power and streetability. i dont think its too big,but it is way biggfer then stock. i ran the 280 mag years ago and still had 10-12in-hg. the increased duration might be why your seeing a fluctuation. like jeff said try a manifold port for proper measurement. some carb ports are "timed" ports for different uses.