Question for ooollllldddd sj.com gearheads

My "other" car is a 1962 Buick Special.

I've had problems with a leaky transmission this summer (pretty sure it's the seals) and therefore have not driven it much recently...to the tune of once every two months or so.

Today I decided to get off my lazy ass and drive it the four miles to the garage to get new seals. It starts up fine like always, but I have no power. Top speed is probably 40 mph and when it's idling, it's pretty low.

It shifts fine (it's a two-speed automatic) and I could drive in each gear but I have very little, if any, acceleration. I sped up when I went down a hill.

My fear is that I need to get the engine rebuilt (I have low oil pressure), which I knew was coming down the road but was holding it off until, you know, my bank account is a little more bulked up. It's a Fireball V-6 (for you gearheads, the first V-6 ever put in a car) and it kind of feels like perhaps not all of my cylinders are firing. But, also, could it be the carb? The gas in the tank is probably about a year old, and perhaps the carb is clogged?

I made it to the garage and added a tune-up to the seals, so I guess I'll find out soon enough. But I'm just wondering what I should be bracing for.