'79 TH350 Crossmember & '63 frame reference

[J1MMY]

Just in the process of offering up the TH350 crossmember to the frame of my '63 and I cannot quite get a snug fit - there is a small gap between the upright side of the crossmember (that bolts to the side of the frame) and the frame itself. Is this usual and down to a slight difference in frame size?

I'm thinking that to get around it I could slot the holes that the transmission bolts go through, trim down the opposite end of the crossmember and push it across as it were - the slots will allow the trans to be re-centred on the crossmember.

Thoughts/experiences?