Fenderwell and using side pipes?
I found a spring shop in Detrioit that seems to be a good choice for when I replace the rear springs on mine. They have original OE blueprints and build to exact or custom specs.
https://www.eatondetroitspring.com/
I'm not buying generic springs again. I've already been there on another vehicle.
As far as 300 performance stuff, check out this crank scraper. https://crank-scrapers.com/Ford_straight_six.html
No idea if it'll do anything below the lowly six's max RPM, if anything at all, but it's gotta add at least much power as a NOS sticker on the back glass.
Something interesting with at least he Chevy 292 and Ford 300 was they also came in industrial versions with forged cranks and some other goodies. I've been told the 2ton truck versions had 7qt oil pans. Some 292 blocks made in Mexico supposedly have a stronger/longer wearing iron alloy (I guess like the big Caddy V8s). My 292 came from a Massey Ferguson soy bean combine; pulled it myself.
The Ford and Jeep US versions got fuel injection before the end. The South American Chevy 250 got fuel injection. Then there's the Australian variants of the US sixes, with more power. I think Dodge done a performance version of their slant six, which may have just been some tuning adjustments or slightly higher compression. Of course, the very first Corvettes had an inline six (shhhh, don't tell anyone LOL).