http://www.georger.com/gds/t5_transmission_conversion.htm
Yes, it is possible and not too difficult but cutting is required. I have had overdrives in my PV for years and driven many miles. This week I was in New Hampshire for a day ... from Buffalo. in April I did the bucket trip around the usa. very pleasurable. As for road noise... it's been so long I don't remember what it was with a stock setup. With an overdrive it is fine. When it comes to B23 FT or B234 etc.... I have thought of these as an upgrade because they are newer. Then reality comes into play and the fact that the B20 is just a great engine. Second is the B23 is Fuel Injected while the B20F/E can be easily retro fitted to carbs. I have had just about every carb on my car (SU H6, SU HIF6, Weber 32/36, Weber DCOE40, and now Mikuni HSR 42) and they all worked great. I have 2 B20F's in my 18000ES's and have dropped the FI in favor of the simplicity of carburation. All the wires are gone, all the sensors are gone, all the poor grounds are gone, all the injectors are gone... gone. Now All I need is fuel pressure and the carb takes care of the rest. I have the B20 in my daily driving 444 and have clocked 80,000 miles. The B20 fits into place easily, the tranny "cut" is not terrible, with a little compression and cam you can have great power AND reliability. For compression I have had head work done and shaved the head to get near 10.0 : 1. Recently I found that an easier way is to replace the stock head gasket that is .040 with a Copper one that is .016. That's the same as taking .024 off the head but it is for the entire combustion chamber and not "in" the head. There's about .6 increase in CR with this process. So, stay with the B20, go ahead and cut the tunnel to fit the tranny (T5's are nice) build the motor with a realistic goal of reliability and good grunt (125 HP is fine and easy to get) go with new Mikuni's to have the luxury of years of development compared to SU's, get a 123 Distributor for the same reason and then enjoy the ride!