I'm leaning towards a spread bore carb to replace the Holley 600 that is present on the imperial... Hoping to get some boost in fuel economy, and off idle crispness in the swap.
A good running 600 will rival the others.
Yours has issues obviously, as defined by it's poor economy and not crisp.
If you buy used, you'll likely need to kit whatever you buy, same as yours.
As for economy, any carb can make economy; you just lean it out until it won't run no more. The Holley 600 has gone over 30 mpg for me, in point to point driving. Jus saying.
Timing plays probably an equal role when it comes to economy.
And crispness comes from cylinder pressure, which is not cheap to change..
I don't think you will ever find an old 8/1 SBM to display crispness. And with factory retarded timing and a rich low-speed circuit , it will never happen. No matter what carb you put on it.
And don't even think about running a spreadbore carb, on an adapter spacer, to fit it onto a square-bore intake. It can be made to fit, but usually requires a die-grinder and proper gaskets. But just cuz you get it on there, doesn't mean it's gonna run any better than the fixed-up 600.
I'm no fan of that 600, I just want you to be aware of your options. I am a big fan of spreadbore carbs on spreadbore intakes for low-compression engines. You bet.
But if your Imperial has super terrible hiway gears, then crispness should not be in your vocabulary.
Wiki has Morrisville listed as 732ft above sealevel, so that ain't too terrible.
If you decide to kit the 600, install a 10.5 PV or something real close to it. And check that the floats are not heavy with gum, and that they actually float.
and then there's this;
HOLLEY REBUILT Rochester 4 Bbl. Carburetor 85R-8961 / 64-70107 | eBay