"...cheapest is a salvage Duraspark..."
that (a Duraspark II) from a junk yrd and a recurved dizzy would B #1. These I6s do not breath very well so yer intake idea is tough (w/our integral log head).
Everything after the DSII depends on end goal/ur specific application (everything automotive to me is application - whadda ya want the rig for/to do). I'm a systems thinker (& employ) and believe that approach needs to be utilized here as well. Can't just slap some "tubes" on & get a hp kick. Also why I mention further research for ya. Come to know these I6s - I have (10 yrs research: 144, 170, 200, 250, 240, 300) & have... (searchin 4 wrds here) a special respect for them, know more of their ins & outs.. So - again:
want a restomod, period correct, show car, racer, daily driver, toy, etc?
Mine's the 1st plus last 2 and a money maker (wrks for income/it's keep).