Where do I start?

[FRANK BASILE]

Plenty of measuring first. Depends on your expertise in cutting/welding to start slicing firewalls If you simply pick up a drivetrain and find engine/trans has fit problems. If this path is chosen then leg room and interior rehash follows on. Similar for width, too wide and you end up with exhaust manifold/steering issues to get around. Another thing to look at is oil pan and your chosen front suspension clearance. For a reliable daily driver choose your desired drivetrain and plan from there. Needed anyway for the engineer first chat. One thing that can happen is that one chooses a drive train and then such things such as steering interference becomes an afterthought. Plenty of choices when you have found others already having done a similar make/model vehicle. Copy proven combos is easier. "Orphans" are different. Keep up the research, ask more questions, and plan first. All can go smoothly then for your final enjoyment. Plenty of projects would have started out on dreams that quickly turn to discouraging nightmares.

[FRANK BASILE]

Small block Chev is approx 600mm from crank pulley to back of heads, after I noted you mentioning 600mm firewall to radiator.