recommended 5 speed gearing

Personally I would stick to the automatic or what I prefer to be semi-automatic. If you keep your stock ratio, a NWC T-5 with the .86 OD would put you at 2550RPM cruise. The .76 OD version would put you at 2250RPM. Both of these are a skip away from the nasty 3K cruise speed I associate with big block engine failure. The low compression weirds me out but I can tell you that you'll want at least 10:1 before small tube headers sounds like a good idea.

I didn't read the article,

I forgot to mention that there are a huge number of parts available for the T5.

and,

Hanlon Motorsports has a video on rebuilding the trans that makes it easy peasy.

I have a 70 Rebel running a stock down to the air pump 390, T5 Z, 235 60 15 tires, 3:54 rear gears. The T5 Z has a 2.95 first, .63 5th.

On the hills here in southwestern Pennsylvania the 2.95 first gear is excellent. Super easy to get moving on a hill without lots of clutch slipping like it took with the 2.20 first gear T10 I used to run. When I've been to flat areas of the country, like Ohio, sometimes the low first gear seems a little too deep. Not to the point where it is bothersome though. The .63 5th is great with the 3:54's, it is so easy to run with the traffic on the highways without the engine screaming like in the 4 speed days.

With your combination the T5 Z gear ratios might be just right for you.

The nice thing about the T5 swap is how many of the factory parts you can use to do it. 4 speed driveshaft works with a yoke swap, speedo cable works, clutch linkage works, it fits under the floor without lopping the top off the tunnel.

Do the swap, you'll love it. It is the single best modification I have ever made to a car.

BDCVG wrote: Just be careful of too low of a 5th gear. In My particular case 1980 rpm is 70 mph. My car pinged when accelerating up a steep enough hill so I had to modify My advance curve to avoid down-shifting. I now live in the hills of N.E.   P.A. and the .65 O/D is almost useless.

That's one reason I went with a 6 speed. 5th is 0.80 and 6th is 0.63. Most of my driving is on secondary roads at slower speeds, so use 5th more than 6th.

For a 5 speed with overdrive, the Richmond Super Street has nice ratios to work with a 3.15 rear gear, with less drop from 1st to 2nd and 4th to 5th ... but not a low-cost option, so the T5 is far more popular. Match your rear gear/tire diameter/cam range and T5 should be fine.
Richmond Super Street Chevy version - works with Quicktime RM-8075 bell
1st - 2.89
2nd - 1.85
3rd - 1.31
4th - 1.00
5th - 0.77