Cams, pistons, and a good set of heads.
Tuned one we built here last summer,(used to build a TON of them years ago) with an old R&M 2/1 header....1000cc's, not a stroker......63 ft/lbs, 65 hp!!....with OE "P" cams!!
Scott

 

I am thinking of a basic valve good and cleaning up the heads as needed, so someone will have the heads and do the cylinder bore. I can assemble everything here at the house

Here is what was suggested to me on another page. 10:1 wiseco pistons and rings, Andrews pb+ cams, rebore, vavle job/clean up heads, thinnier copper head gasket

 

one thing for sure, the drags are a thorn. not to say you can not get them to run but it takes a lot of experimentation to dial L>H to play well together.
my 1974ch with p cams, dialed in 2" drags running redline 7.5k @133mph was pretty cool for stock.
about 4 years ago with 150k on the clock, i decided to do frame paint, bird in hand, upped the ponies.
running 10:1, y cams, 1.750 long drags (nightmare story here) compu-fire dual mechanical adv ign still runs to 7.5k and still around the 133mph mark (in the gearing, just gets there quicker)

good exh are hard to find for the iron but if you can find a set of 2 to 1"s, will help a bunch especially 5K up range.
reversion will be an issue. i tried several baffles and could not get low and high, either or!! so i whipped up a new system and it worked great for me, i call it a twisted baffle, i also have a wave baffle idea. i studied industrial and performance systems and a lot uses change of flow direction and not baffles, perfs or chambers.