Valve seats are "as cast", not removeable OEM. You can opt for "hard seats" for better unleaded fuel compatibility, which is an additional labor operation. Which can lead into the 1.74 exhaust valves, with a bit of additional machining time. THEN get the guides upgraded to the bronze heli-coil style of valve guide inserts for even better long-term durability, but even the old "knurling" of the guides worked back then. Then good, OEM-style valve stem seals.
"More power"? Off-line, passing/mid-range, or 500+ top end horsepower? "Bigger cam" in the realm of the 383/335 and 440/375 cam is a good street cam. Lunati has a good one which another member has in his '68 383 4bbl Fury. Good pricing, too. BUT any cam upgrade will need the OEM 4bbl springs to match, typically. THEN comes dual exhuast with good mufflers (not the loudest ones, but the ones which have better flow). BUT for a basically stock engine with some upgrades, 2.5" pipes will work just fine with plenty of capacity.
The Edelbrock Performer 440 can be a good street manifold, too, matched with a Edelbrock AVS2 650cfm carb. With all of that fired-off by a good electronic ignition system.
You'll probably need a slightly looser torque converter, too. Chrysler used "tight" torque converters back then, which worked very well with the stock cams back then. But someting along the lines of the '68 Road Runner 383 (which was also listed as a Slant Six 225 torque converter in the Chrysler reman converter listings) would be a good compromise with a little bit more stall but not too loose for efficient highway driving, by observation.
By the same token on the heads, finding a good set of later 906 castings (which already have the larger exhaust valves standard equipment), getting them worked with some hard seats might be a better alternative to working yours. I suspect your heads have the 6-bolt valve covers? If so, the 906s are an incognito upgrade.
I mention the '68 Road Runner 383 torque converter as a reference item. "Stall speed" is relative to the amount of power in front of the converter and is a variable situation for a given "guts" in the converter, rather than a "hard and fast" spec. Many higher-stall OEM converters of the later 1960s were actually original-application 6 cylinder items, by observation. Torque converters for a 413 car engine might not be the same as a later 440 car engine, in how they attach to the crankshaft/flexplate, as I understand it. Others can elaborate on that.
Trying to transfor the car into a tire-scorching monster might not work too well with any stock set-up per se, BUT you can have a better performing vehicle with a better intake manifold (compared to the stock ones of that era) and a better carburetor (compared to the stock ones of that era, whjich leads to the Edelbrock items mentioned above), with a better flowing quiet exhaust system. Granted, more noise can give the impression of "power and speed", until you watch the speedometer needle not move much quicker.
Right now, rather than focusing on "more power and speed", get things back to OEM specs and also do similar things to the other systems on the car. As in rubber items -- suspension bushings and ALL fuel lines, front to back. Get it cleaned and spiffed to as good as it can be with optimized performance with the OEM set-up. Learn about it and how it feels/reacts to normal driving situations before starting to seek to "make it better".
No real need for "big name" ignition items. An OEM Chrysler electronic set-up is one of the best for durability and performance. Sourced from Rick Ehrenberg on eBay, for example. Add some NGK Iridium spark plugs (with good spark plug wires) and things should be in the realm of "lifetime" rather than "every 12K miles", as when the cars were new. Especially considering what it takes to change plugs on most Chryslers! FWIW
Just some thoughts,
CBODY67
