Kev, There is no real header, intake and carb package available, but you can piece together a more effective combination. You can also put together a combination that is is performance robbing.
There are no long tube headers made for your Riv. Shorty headers are available from TA Performance and Sanderson for the 64-66 Riv. There are also a few others that make header parts made for the Nailhead if you want to make your own.
There are some dual quad aftermarket intakes made for the 401/425, but those currently available and in production are more restrictive then the stock manifold. The B262 Edelbrock and a later copy of that manifold by Eelco were very good flowing dual quad manifolds, but be prepared to pay $500+ for one of those when and if they show up. There is also the stock dual quad manifold that came from the factory. These are again rare and a little less expensive when found, but will increase performance with the right 750 CFM carbs and tuning. There is no single four barrel aftermarket intake manifolds made for the 401/425, but some of the stock manifolds are better then others and can made better. The 1966 single four barrel made for the Quadrajet (on all 425s and some 401s) was a very good carb and intake combination that can run as well or better then the factory dual Quad. This is usually a somewhat cheaper option then the dual quads and is easier to tune. These again are rare, but can be found for less then the dual quads as there is less bling factor. Lastly you can have your current factory AFB intake machined to fit a holley double pumper. The bores and attaching points need to be opened up to use this carb on this manifold, but an 850 DP will wake up the Nailhead. You just need to find a competent machinist over there.
You might as well do a mild cam if you have gone to trouble above. That would take advantage of the increased intake and exhaust potential of your upgrades. Although, you can not go too large on the came before needing to change the stall of the transmission converter.
HTH -
Bookmarks