2bbl Carter on stock manifold

That's a tell-tale sign of a good-size vacuum leak, almost irreplaceably from the throttle shaft bores being worn. Either get a remanufactured 1920 or upgrade to a Super Six. You should've kept that Offy manifold, but put the smallest size Holley you can buy (I think 390 cfm) or something like an Edelbrock 500 cfm (a little big but only flows as much as the engine needs). If you really want to open up your slant I'd recommend "Dutra Duals" to keep your choke and carb heat or headers if you live in a warm climate. Add a good 2 1/4" dual exhaust with H-pipe and that thing would go.