OE AVS's of that era 1968-mid 71 are extremely lean & as Bee said converting to the short Edelbrock performer primary jets/2 step metering rods is mandatory. & besides the jetting the "passages" may also be less than ideal for performance (I cant say as I ain't a carb guy). No knowledge of thermoquads & the tquad guru Demonsizzler in not longer in operation. Just me, you said heavily modified I would go with the new holley spread bore I forget its name (demon or avenger one of those two) & it gets multiple good reviews on here & it would require as said the adapter spacer for a square bore intake. EDIT not sure if it is spreadbore or square