You have nothing to worry about, unless you plan on taking the body off without a firewall and rolling it on it's side.
Honestly, you are breaking the first rule of rodding, trail fit EVERYTHING! I would never even think about welding that firewall in if I hadn't fit the motor, no friggin way. Listen, you ARE doing it properly and no steps back, you have fit the firewall, it's screwed in, you have taken the proper steps, now put the motor in and take the next step.
My very first job I ever had was at a rod shop after school when I was 17. I was taught this early when my boss apparently didn't trial fit ANYTHING. I will never forget him assembling his blown small block powered 29 Highboy and putting the chromed headers in to find that they hit the scatter shield, and I mean hit it bad. And of course things like that always are deceiving on how MUCH it's hitting. So he tapped it in, not enough, tapped it in, not enough, he ended up with a smashed pipe and still not enough and put stock exhaust manifolds on it! A blown motor with stock exhaust manifolds.
Next was the front axle, he had all the axle and components chromed before trial fitting! The friggin tie rod hit the frame! So there he was dropping the spindle steering arms on freshly chromed spindles! That was my first lesson about the akerman angle, we discussed it and I have never forgot.
But the biggest lesson I learned was to trial fit parts, EVERY part.