I have a 90 740 turbo (B230FT) that has strong gas fumes in the coolant reservoir. It doesnt seem to use coolant. I'm getting ready to replace the head gasket. How likely is a cracked head or block? The car has 203500 miles on it and runs great.

Obviously it is hard to know if head is cracked or warped until you get it off. If you see an obvious blowout of the gasket from cylinder to water jacket then that would be a good (but not for sure) indication that head is OK but check it for warp. Head can be checked by some machine shops for warp and crack once you have it off. Highly unlikely that that block is cracked.

Any other symptoms: coolant in oil, oil in coolant, done a compression test on cylinders and pressure test of coolant system? Did engine get overheated?

I had several overheats on my 88 740 with and my head was cracked. Purchased a known good head used from a Volvo shop that I trust and it worked liked a charm.