Are the older trannys built better,sound better than those made today ...
They're in the end just like transformers today. The expensive ones were very well made, and there were cheap ones that were made to a price-point. Some manufacturing techniques might be different, but someone, somewhere makes a transformer today that will match (or exceed) the performance of any of the old transformers.
I think the art is in selecting the right modern transformer for the intended application. Regardless...
What has been your overall take on using older transformers from donor amps?
Your safest bet is to re-use/re-purpose the existing chassis/OT/power supply/output tube set of the donor chassis in your new build. OR, take
very meticulous notes on what amp, what supply voltage, what color code, what connections, what tube type, etc., was used in the original donor chassis, and keep those notes with the transformer. A lot of folks have suffered needless heartache trying to figure out the characteristics of an "unknown transformer" they took from somewhere but didn't take notes on how it was being used (or lost those notes).
And instead of having to try to reinvent the wheel when you use the old OT in a new amp, you know that it worked well with the original circuit, tubes and supply voltage. Which is also a good argument for leaving the entire output and power supply section of a donor amp alone. At least then you know it should work properly if the individual components haven't failed.