hello new to the site i have a 67 Belvedere, 383, 727 auto, i have done a comp' test and have 148 lbs on all cylinders except # 3&5, 3 has 130 5 has 142, i understand that 15% difference is acceptable, other guys say it's 15 lb, I have cracked 2 valley pans within 1500 miles, i also am leaking out of the timing cover, gaskets been replaced 2x this year. oil blows out of the dipstick tube as well. could this be associated with blowby on the 2 weak cylinders  .  

Definitely,blowby! It also sounds like your block is flexing ,indicating it is cracked somewhere. The engine has to be fully stripped and thoroghly inspected.At the very least I'd say a rebore with new pistons or resleeving of no's 3 and 5 at the very least with all new rings and newly torque plate honed bores.Or find a second 383 in good or rebuildable condition,keep yours if its numbers matching.  

comppression test is checked by a % not lbs. 15% is acceptable but i dont like to see anything more than a 10% difference. i have seen a sbf with 20% and still run fine. if it is not cracked and you do bore it you should also have it line bored. motors do warp. this is kinda rare but ive seen it before--check you motor mounts. if one is broke or really loose it could have caused the block to warp over time. hpoefully you just have a bad timming cover and worn cyclinders..