Going to buy a set of 15x7 rallys for my 70 RR. Plan to go with just some Radial TA's for now. Whats a good size?

Got 255/60's on my '68 GTX....lots of room and they fill the wheel well nicely.
Anything bigger and it gets a little too much for the rim width.....I tried that.

tall or short ? you could run 265 or 275 even 285, 70, if available, they used to be, if you want taller than a 60

I have Eagle GT IIs and 255/60s look good. I've tried 275/60s and they look fine on a 7 inch wide rim even though 8 inch is recommended.

T/As may be slightly different for height and section width but it would be minimal.

Mike.

Front or rear or all around? (did I miss for front or rear?)

225/60-15's on 15 X 7 on the front.

255/60-15's on 15 X 8.5's on the rear.

The rear is close but still has room with original sheet metal but 275/60-15's on the 8.5" rim will rub for sure on my GTX

As far as fit to rim, the tire manufactures have a recommended rim width and you'll get the most performance from a tire by staying within it.
For a 7" wide rim generally you'll be looking at 215-235 in the metric width tires.
As far as sidewall, that's really up to you. Shorter sidewall will have less give and generally feels more responsive and corners better on good pavement. Taller sidewall can keep better contact and better protection on bad pavement and off pavement.

Remember if you change the rolling radius from the current tire/wheel combo, that you'll likely need to change the speedometer pinion. Not a big deal, but worth noting.

An alternative to the BFG T/As are Firestone's Indy 500. Same catagory of tire, but a little more grip according to Tirerack's survey. I agree, but actually give less weight to my opinion because the T/As were old and seemed to be getting hard when I switched.

One vote more for 255/60-15s are the best match for them. Maybe 245/60-15 too

I ran 275/60R-15 BF Goodrich radial TA's on my 69 RR for many years. It's the biggest I'd go and they worked great for what they are. Meaning, TA's are a budget street tire that is too hard for any good track times but great for a street machine cruiser.

Went one size bigger to the old Mickey Thompson "Letter I" bias ply's that were L60-15's and they didn't want to sit flat on the pavement unless I went way down in tire pressure (like 20 psi).

Personally, the bigger the tire in the back, the better your B-body will look!

These are 255/60/15 on all 4 corners on a 15x7 rim

31-10.50 x 15

R.