If you build the 396 you would have to spin the motor a little tighter and run a little more cam and compression to bridge the 454s advantage in cu inch and Torque!
The 396 has had a big reduction in the cost of its parts. A forged 396 crank sells for about $100, A forged crank for a 454 could cost as much as $400 , And a used 396 short-block will usually only set you back less than $300. While a 454 short-block would cost $600 . You already have the 396 so you can put the 454 money into your little RAT build, and with a 450 to 500 HP 396 that revs 7500 RPM , put the 454 boat anchor back on the chain and in the water! Its not that hard!
After building and running 409s, I owned and built many 396s & 427s mainly blue printed & balanced stock 396/375 HP & 396/425 HP & 427/425 HP high revers that when built just about dyno'd at 450 & over 500 HP. And that's stock blue printing and good balancing! A little porting ,and L 88 cams(L-88, 264/270, 112/108, .560"/.580" lift) or the ZL-1, 262/273, 110/?, .560"/.600" lift) but there are much better solid roller cams now, and intake & carb work, I don't have much experience with the 396/325 HP builds except for numbers matching restorations a few street rod minor hop ups! but installing splayed-bolt caps like P-Body said cures the main problem!
If a 454 is good why not a 496
Mr. P-Body, And I both seem to like 396s & 427s LOL we're probably a dieing breed just like the "Little Big Block 396s" I think the heads you have would keep your compression up and with the stock pistons the compression was around 10.5 to one, a little milling, a deck shave, a thinner head gasket and 11 to 11.5 is possible, do your own porting, a ZL1 cam or solid roller, Roller rockers, lighten the valve train, balance the reciprocating assembly, etc and 420 HP is reachable!
Better heads (aluminum) & pistons for Higher compression (13to1), bigger cam and 500 hp can be achieved but fuel requirements will be hard to feed on the street!
Why did you pick the figure of 500 HP for the 396? AP was right about its more easily obtainable with the 454 (and on pump gas)! The 396 /425 HP little rat when you put your foot in it would;> Still Snap your head into the back seat