If you have a Chevy 2 speed PG transmission it will have a 27 spline output shaft to fit a driveshaft with a 27 spline input yoke.
Other transmission output splines ( I did not list all the options)
350 Turbo = 27 spline
400 Turbo = 32 spline
700R4 overdrive = 27 spline
57 to 63 Borg Warner 4 speed = 16 spline ( normally called coarse spline)
63 to 70 Muncie 4 speed = 27 spline
70 to 74 Muncie 4 speed = 32 spline
Richmond 5 Speed and 6 speed = 32 spline
Also a PG transmission is the same overall length as a 350 Turbo transmission
BW & Muncie’s listed above are the same length as a PG
What plugs are you using?Try your timing at idle at 8 degrees before top dead center,are you using premium fuel?Double check that you have a ground strap between block and cab or frame for proper ground.
Jamie,the current transmission in your car isn't original to your car since the 58 powerglides were cast iron.If your looking for an answer about the engine,it simply doesn't exist. The vin will only tell wheather the the car came with a 6 or 8 cylinder engine.There's nothing on the data plate,and partial vin's on the engine didn't start until 62 beginning with the 300 horse 327 engine.I 'm not saying that it is or isn't just that there is no 100 percent way to prove that it is.