If I'm reading your goal correctly (I might not be), seems like you want an overdrive transmission. ("not going anywhere soon" comment - I assume you mean your rear gearing and the 1:1 final ratio of the SM465 make highway speeds untenable?)
T10, Super T10, and old car Muncies will have the exact same performance on the highway as your current trans as they aren't overdrives either.
The O/D 4-speed Muncie you mention is indeed very hard to find. Had a chance to buy one about 20 years ago and wish I had done so. Oh well...
These guys make aftermarket Muncie O/Ds that might work for you since you have buckets - but I am not that familiar with the company.
http://www.autogear.net/motorsports/
T5 is a 5-speed, 5th being the O/D. It's not exactly a heavy duty option, not as strong as your 465. If you baby it and aren't doing burnouts, it's a nice trans, swapped one into a 66 several years ago.
http://67-72chevytrucks.com/vboard/s...d.php?t=482069
Personally, I think your best bet is the NV4500 - great gearing w/overdrive and it's pretty heavy duty. Several threads exist on here about how to swap one into a 67-72, just google at the top of the page. But here is one of the best:
http://67-72chevytrucks.com/vboard/s...d.php?t=673588
There is no spline associated with the engine - you simply have to have a clutch disk that accepts the input spline count of the trans - and you need a front driveshaft yoke that matches the output spline count of the trans. If you're considering swapping the trans, recommend install a new clutch, pressure plate, input shaft pilot bushing (in the back of the crankshaft (roller bearing pilot bushings are a little over-rated, if you ask me
), and inspect/resurface/replace the flywheel as needed (replace if any signs of heat-related hairline cracks)). If you don't have one - a clutch alignment tool (of the correct spline count to match your new trans input shaft spline count), is a worthwhile investment, even if you plan to only do this once.
If I've misread your goal here - and you just want to know if a Super T10 will work - yes, it will. It's lived behind millions of Gen 1 small blocks and is a good trans. Car muncies will bolt right up as well, as will a T10. But NONE of these will improve your final ratio highway gearing - they will have the exact same performance as the SM465 at highway speeds - so there's really no incentive to switch to one - and you have to cut a new hole in the floor for the rear/left mounted shifter.
If you can elaborate a bit on the reason for why you want to replace your SM465, we can probably help a little more.
Good luck!