Are rebuild parts/ service parts readily available for these? I don't want to upgrade this car to anything more than the 7 1/4 rear that is in it now but would like to find a sure grip for it. It's a daily driver and will have maybe 130 wheel horsepower with the mild head work and upgrade to super six.

As far as I know, service parts are not available for the 7 1/4 SureGrip units.
DD

Like Doc says. The only way to find SG parts, is if someone has them on the shelf somewhere. The last set of clutches I ran into, the seller wanted around $150. I passed on those.
I do have a spare set, but not for sale at this time, as I do run a 7 1/4 SG rear in one of my race cars. Maybe someday.
PS: there are two different style SG units for the 7 1/4. The early clutch style with 4 spider gears, and the later "cone" style with only 2 spider gears.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Thankyou guys!
Charlie, which is better?

The clutch style is better.
DD

if it is the cone type non rebuildable,they can be fixed