Block number 3999289 is out of a '72-75 passenger car or up to a '78 light duty truck (most likely the latter) it was only issued as a two bolt version. It has the short distributor tower inside the lifter valley and is identical to the 427 block number 3963512 it replaced (only difference on the engineering drawings is cast relief's for the 454's 4.00" crank. To know when your block was cast look on the rear of the block on top of the bell housing mounting flange for the casting date code. (Three or four digit code with a letter, and one or two numbers, and a final single number. These were used to represent the month -- day -- year, using A through L for the month: A=January)
As to your heads; L18 73 is the casting date and translates to December 18, 1973. The GM 10T says they were cast in the Tonawanda, NY foundry. PASS stands for "Oval Port" not the diminutive round "Peanut Port" found on modern BBC castings, but the larger 256cc intake with 114cc square exhaust ports equipped with 2.06" intake and 1.72" exhaust valves. Your 336781 heads are one of the better ones having cast 113cc open combustion chambers rated at 235 net horse power in '74 (270 horse for the same engine in the 'vette because of a better exhaust system). They were used exclusively from 1974 through mid year 1975 in all 454 engines.
Your heads will really respond to a pocket port and larger valves. All big block Chevy's are exhaust limited, so the 1.88' dia. stainless steel swirl polished exhaust valve with a three angle valve job really perks it up (note the difference that just the use of dual exhausts with large free flowing pipes found on the Corvette does for the same engine as yours when not stuck with a single exhaust PU truck pipe). The addition of a 2.30" intake can also make a world of difference. If you do rebuild the heads with larger valves replace the guides with 11/32nd inch bronze inserts to reduce friction.