318willrun, well you have kind of stated my situation once I bought my 340 Duster. It had a running ‘74 casting 318 in it with no vin stamping and a tired 904 that needs a rebuild. Whoever put that engine in kept the 340 exhaust manifolds, distributor, and harmonic balancer.
I bought a long block disassembled 340 (‘69 vin) for $900. Block is standard bore, cast crank, pistons are 30 over, heads are factory 2.02 intake valves, oil pump and pickup, windage tray, oil pan, and timing chain cover. Nothing else.
I later bought a complete running 340 (‘71 vin) for $1200 with all of the accessories attached (alternator, water pump, and b-body exhaust manifolds). This engine is 30 overbore with 576 heads (smaller intake valves), an SP2P318 intake, and a Holley 4 barrel. The numbers matching 727 auto transmission is still attached.
My original plan was to beef up the 318 while I had the machine work done on the basket case 340. So I purchase a used Edelbrock performer cam and lifters ($125), used Edelbrock Performer 318/360 intake ($100), and a used Edelbrock 600 CFM carb ($100). Later I came across an Edelbrock LD4B for $175 and got that.
Meanwhile for the 340 I picked up a factory ‘71 intake for $100, an LD340 for $200, two different cams (a Hemi grind, and a 484 purple shaft) for $200 each, an HEI distributor, and a few 340 ThermoQuads (1971 ($275), 1972 ($75), and 1973 ($50)).
I also bought a set of drop in solid roller lifters and a set of 273 banana groove lifters if I want to go solid roller cam.
Once I came across and purchased the complete 340 and trans I have been wondering if I should waste my time on the 318 upgrade while trying my hand on working the heads per your tutorial or should I just go ahead and spend the time making the ‘71 340 into a better version than what it is.