Would putting a 90 degree fitting on the back of the hardin stainless pump restrict pressure too much ? I have a Gen 5 pump but the water inlets come out the side of the pump for clearance with the motor mounts. I'd like to upgrade to the stainless pump if possible.

You can't put 2 90* fittings on unless one sticks out completely past the other and then you would probably have clearance issues.
I would not worry about flow or pressure issues.

Ck Teague for radius or mandrel fittings .

Yes Teague has a long and a short 90 that I believe might work. I'm going to take some measurements and see if they will fit. Just wanted to see if there would be any pressure issues before going down this road.

i have one 90 and one straight on my hardin stainless pumps. have not noticed any pressure loss from the fittings.

I have a straight and 90 as well on the Hardin Stainless, been that way for almost 4 years. Seems to work well.