sunk valves, stem height problem

Stock heads:

Bigger valves = NO - SBC = 1.74-2.02", 273/318 = 1.78” 340/360 = 1.88-2.02", Exh SBC = 1.5-1.6", 273/318 = 1.5”, 340/360 = 1.60"
Thinner stems - yes & no compared to 273-360 LA 3/8 yes, the 318/360 Magnum at 5/16" is thinner than SBC 11/32". LA valves are stepped down below valve guide.
Bigger ports - Not really I have a 350 in my boat (for now) with 461 double-hump heads, Intake ports visually appear about the same as 360J heads, exhaust at least appear bigger on J heads (maybe not be any better flowing though)
Integral seats ( both intake and exhaust) vs integral int exh insert in the mopar. Not always, Pre unleaded fuel both had integral seats, after unleaded was introduced insert are best for long life (not sure if/when Mopar introduced them in USA, In Australia integral seats went through to 1981, Same with Chev, not sure when/if they went to inserts on cast iron SBC heads).
Nicer cast less flaws - externally perhaps less casting flash present on the Chev – not really a flaw, just cosmetic.
Stud mounted rocker arms and all the otjer things everybody knows - Not an advantage, when Chev's want to try and rev high with any sort of reliably they mostly convert to shaft rockers. Also SBC pressed in studs are renowned for pulling out of head with heavy springs, Chryslers stock shaft rockers are far superior to stock chev stud mounted rockers.

SBM valve angle = 18 deg, Chev 23deg, Chev aftermarket heads often go to 18 or less deg to match Mopar's stock valve angle.


Comparing your probably stock valve size 318 heads to perhaps modified 350 heads is not a fair comparison. Compare them to stock 340 2.02' heads.

Cheers
Jamie