need help ID'ing bbc by casting numbers

I bought a 1981 k20 4x4 chevy with a big block in it. The engine is not stock and I was told by the seller that the engine is a 454 create engine. I pulled the casting numbers (3999290)and found it to be a 402 not a 454. The head casting number is(3917215). I found that they are closed chamber, is this good or bad?. The plugs I pulled out are r45xls, are they correct for the Head? I am very confused about what the heck this thing is, and want to find out so I can do a proper tune up and maybe do some small performance upgrades. This is my first bbc and im a little lost on this one.

vin #on the engine is( T01I2CTB ) ser #on engine is(12L5451472282:confused:)

45 plug is a fairly hot plug, depends on the compression ratio and cam. Try and find out if the heads are original to this block and what the engine originally came in as a base line.

K204X4: What you've got, is a 1972 BBC 402 block, as you stated. (Identical to the venerable 396 engines, except for a 0.030" larger bore, beginning in 1970). That engine production code (not "vin") breaks out as follows: T = block was cast in Tonawanda, NY (where all of the good, high-performance chevy engines were manufactured in the 60's & 70's). The next 4 characters, "0112" (not an "I", even if it looks like one), are the month and day, as in January 12th. "CTB" is the identifier of the engine configuration/HP rating, and it's intended application/vehicle. ("C" for cars, "T" engines went in trucks. "TB" was a 4-barrel, rated at 240 HP). That version of the 402 was installed primarily in Chevelles, Camaros, Impalas, and some Monte Carlo's - all with automatic transmissions. BTW, don't be fooled by the HP rating - the same engine would have been listed as 330-350 HP a year earlier, but GM switched to the SAE rating system in 1972, which lowered the numbers by about 100 HP.
Your heads are 1966-69 castings, from either a 396 or 427, and are closed-chamber, oval-port heads. The CTB engines originally had open-chamber (O.C.) heads, and 8.5:1 compression ratio (C.R.), so I am guessing someone replaced them with the tighter-squeeze heads to get the C.R. back up near the 10.25-10.5 range that the earlier BBC's had. I don't know if they rebuilt the engine, or replaced the pistons, because many O.C. pistons are not compatible with C.C. heads. Also, today's pump gas, (even 91 octane premium), generally will not allow for C.R. of more than about 9.6:1, unless the ignition timing is severely retarded to the point where you're giving back the extra power from the higher C.R. So the engine may have flat-top pistons in it, which would put you back to around 8.25-9.25:1 C.R.
I hope this info helps, and I can tell you that the CTB engine has GREAT potential for performance, my drag race car ran that same engine for over 10 years, with stock O.C. "781" heads, some forged 10.25:1 pistons, a basic hydraulic cam (.553/.578" lift, 234/244* duration), cheap Hooker 2" headers and a Holley 750, and it pushed a 3750# car into the low 12-second range in the 1/4 mile, (12.30 @ 108 mph).
Keep the oil clean, use good quality like Valvoline, use a good oil filter (AC-Delco or Wix/Napa), let the engine get fully warmed-up before you run it hard, and CHANGE THOSE PLUGS to an R44 XLS for cold weather and mild/city use, or an R43XLS if you do any sustained high-speed/highway driving (20-30 min or more at a time), or see really hot weather like we get here, (probably o.k. with a 44 plug in WA). It should serve you well for a long time.