The SM465 is a synchro-mesh granny 4-speed, the successor to the SM 420 that ended US production after the '67 model year. They're about the same. The 420 has even lower gear ratios in 1 and 2, but I can't quote them offhand. If you don't like your 420, you won't be thrilled by the 465 either. I like mine because it's in a TRUCK, not an overstuffed elcamino, but a go anywhere, do anything, heavy duty bush stomper that's OK in traffic too. The 465 is pretty bombproof. (Except maybe for the synchro rings.)
As far as mileage: the jury's out. I used to get about 12 mpg on the old engine. Breaking this new 292 in , I was getting less than 9 mpg. After my buddy [who has a lot of experience tuning Carters and Edelbrocks] replaced a couple of metering rods and fine-tuned my 1404, my data showed I was getting 18.4 mpg. I don't believe that. I must have made a math error somewhere. I will try again.
I used to run a Holley 390 CFM. It was a good carb and closer to the ideal 400 CFM the charts show. It was over 27 years old, and I was tired of dealing with popped power valves every time it backfired. I found it too complicated to tune solo. Carters are easy, I find. Carter doesn't produce their AFB anymore, so I got the Edelbrock. Carter used to make a 400 CFM AFB, but Edelbrock isn't marketing anything that small, so I have to lean out the 500. Since the AFB/"Performer" design is a vacuum secondary design, I hear, it won't let you dump too much gas in. But I'm no carb expert.
Hope this helps.
