Just use the search function or google. There are many threads and photos on OF and rolexforums and vintagerolexforum on the topic. Here is one: https://omegaforums.net/threads/rolex-pepsi-bezels-red-vs-blue-back.19710/
A decent red-back insert, which is the correct one for your watch, will cost more like $2k, more or less depending on condition.
I wouldn't put a blue back on a set like that even if it was free. It's as if putting a modern DNN on a gorgeous Pre-Moon Speedy, just wouldn't look right. Here is my MK2 for reference, judge for yourself.
This is an ok price, not too good, not too overpriced. If I were you I would follow some auction on ebay.
@sneer Your current black insert is also a service insert (or a swap-out), so if you're OK with that then the blue back should be fine as well, no? But at the end of the day your 1971 GMT left the factory with a fat font, red back insert.