Hard tail pulled in.

I had the TC bros hardtail welded on to my 1979 XS650. I had a spacer brace cut to the exact 8.5" the same as the stock swing arm, and clamped in, during welding.
However when I removed it, the frame sprung inwards, and is now 1/4" too narrow.
I have my bike jack in there now, pushing it out to 8 3/4" to allow for it to bounce back a bit. (Hopefull to the required 8 1/2").
She is still straight.
How long should I leave it, or am I wasting my time?

I don't believe time is a factor. Spread it, take the jack out, measure. If it's still too tight, spread it a little further, take the jack out, measure. Rinse and repeat as needed.

You need to be REAL sure you haven't changed the centerline - ie, spread one side further than the other in relation to the centerline of the bike.

Might not hurt to flip the jack between attempts. IE, ram to the right first time, ram to the left second time. Not sure it would help, but can't hurt.

heat the welds with an oxy acetylene torch that will keep the spacing. Allow to cool slowly.
Best to get the wheel centered on the frame so the sprockets align, measure and make the correct size spacers.

In case anyone else is looking here; with the way these get welded some pulling in at the axle plates when the welds cool is inevitable, a slightly wider than the desired dimension spacer would have been a good idea, say 1/4" to 1/2" wider....
As the OP knows getting your parts into a too narrow space is hard but sucking in a slightly wide gap with the axle nut, easy as can be. With those long thin unbraced tubes that rear will wander around a bit in use anyways.

Just to expand on what gggGary says.

When welding these plates in it is also a good idea to do a small weld and let it cool before doing any more. This will also help to eliminate to much heat getting into the project that will cause it to shrink and move as it cools........small increments at a time let cool and again..........

If you wanted to straighten without using heat, (as your trying to do), the side that is good has to be braced and the side to shift has to go past point/measurment needed so when it springs back it should be where it is wanted..............there is always some spring back, that is why you need to go past the point, that is the measurement, that is wanted on the finished product

Thanks to all the replies..
I took it out, and it bounced back again, but not as far. So I turned the jack around, as suggested and it out a lot further....we shall see...

Get a foot of threaded rod, a couple of nuts and washers. Put it together screw both sides against each other. And Bob's your uncle....

tim

Thanks to everyone, that answered....
I kinda took a piece of info from each answer....
SHES IN AND STILL STRAIGHT!....PHEW.....