250 is constantly giving me fits... considering something different

[71ChevySS(straight6)]

I'm getting so fed up with this 250. Now it's acting like its got sticking valves or possibly collapsed lifters. Idles fine, revs great in neutral for whatever reason, but put a load on it and it goes to sputtering and hesitating. Can anyone point me in the right direction here? I poured seafoam into the carb while it was running and put a dash in the crankcase, so we'll see what happens...

In the meantime, I've been considering swapping something else in more and more - at least until I can afford to properly go through and build up the 250 some day in the future.

Was thinking big block... but don't have a big block budget. Max $300.

Then I remembered those undesirable 366 and 427 tall deck engines.

Would any of you happen to be within 150 miles of me (McPherson, KS) with an old grain truck sitting on your back 40 that you don't use, but she'll start with some gas and a fresh battery?

If it's still in a truck I can pull it.

The bellhousings on those are a little different, though, aren't they? I'd have to take the truck 4 or 5-speed with it...

Thanks in advance.