I don't think the 700R4 is any larger than a TH350, the TH350 in my '50 Fleetline fits with unmodified floor/firewall
If you have the 700, use it. If your looking for an OD trans, the 200r is bout the same size as a th350
Look for a later model 700R4 because they had internal upgrades that make them more desirable. Then they became the 4L60 .......same trans different name but with all the upgrades. THEN they became the 4L60E...the electronic version. You don't want that because it requires computer control. Its important to get the throttle valve (TV) cable adjusted correctly or you can burn the trans up pretty quickly from slippage. Best way is with a pressure gage. Check Utube for videos.